As promised, I am going to try and answer your question, and support my answers with evidence from my particular background: psychology. I have no doubt that someone will jump in here and ask me to provide links to prove what I have to say, so I want to make it clear at the outset that what I post in this thread is based on my years of education in educational psychology, cognitive psychology, developmental psychology, and clinical psychology, as well as extensive study in linguistics, sociology, and anthropology. I have spent years applying what I have learned to the issues regarding the deaf child and the deaf adult. Therefore, what I post is the sythesization of years of education and experience, and is my knowldge and views, not the knowledge and views of another that I am blindly quoting. Therefore, I am not obligated to cite the sources of the information that is mine, nor am I obligated to provide a link to any specific article, research report, or textbook. That said, here goes.
Mainstreaming affects children differently depending upon their age and grade level due to the developmental stages that children go through at specific times. During the pre-school years, the concerns are language acquisition that will be sufficient for them to develop operational thought at a later stage. Children aged 2-7 are preoperational. This means that they are untuitive rather than rational, and that through that intuitiveness, they begin to represent things internally and do not have to act out their thoughts. If they are deprived of language, they are unable to develop this intuitiveness. When a child develops language naturally, they intuit its uses and its rules. For instance, a child who has acquired language in a natural way will understand that it is possible to use that langauge to get the information they need from their environment. A child who is language delayed will not have developed that intuitiveness, and will use action to gain the information they need. That is why so many children who are language delayed are seen to be impulsive and hyperactive. They are attempting to gain knowlege about their world through doing rather than through thought. Language acquisition is largely a passive activity for children. They learn it by being exposed to it and intuiting its function and its uses. If we don't provide an environment for these children that allows them to acquire language passively, but instead make it a struggle and a constantly active endeavor, we overstress them and impede their abiltiy to learn the way they are developmentally capable of learning. This applies to mainstreaming in the following ways.
A child in a mainstream environment is exposed, in most cases, to oral langauge only, For a hearing child, this does not present a problem. He can hear the teacher's words, and the words of his/her classmates, while his/her attention is directed to a book, or a bird outside the window, or any other distraction. In that way, he is learning peripherally, through exposure. He doesn't always have to give full attention to understand and learn the meaning. A deaf child, however, in an oral classroom, does not have this luxury. He must always have his concentration on the teacher, he does not get the other things going on in the classroom that the other children take in without effort. If something happens in the back of the classroom that could assist him in understanding, he misses it. This child is prevented from learning in the way that is natural for a child to learn. He is over stressed, and sees learning, not as enjoyable and natural, but as hard work. He becomes frustrated, and is quite often labeled a behavior problem.
At this stage, however, peer relationships are usually not much of a problem. Children at this age are very accepting of differences, and once explained to them, they go on to things that are more important to them...such as playing tag on the playground. They are not so concerned that their friends do everything in eactly the same way they do, as long as they are able to interact in ways they find meaningful. In other words, kids this age don't really care if their playmate can hear, or if he signs with his hands instead of speaking with his mouth. He can still play tag, and he can still throw a ball, and that is what is important to them. Interaction. Differences are of no concern.
However, the child that has been impeded in the mainstream setting at this age will carry those linguistic and educational difficulties with them and they will become compounded as the child grows and attempts to navigate not just the more difficult curriculum, but the changes that occur with moving into different developmental stages. As problem solving becomes a large part of his/her learning activities in the classroom, he will show difficulty. Efficient problem solving is dependent upon having completed the developmental tasks of the properational stage. Without the language necessary to allow the child to complete the intuitive prerequisites, he will begin to have problems in this area in the classroom. These difficulties continue to compound all through his/her educational career.
Psychosociall, there are changes happening as well. By about age 6, children begin to develop a sense of self worth through interaction with their peers. As they get older, this influence becomes more important than parental influence on a child's self concept. At the younger ages, chidlren are still very ego-centric (that doesn't mean selfish), and they are unable to empathize by considering how another person might feel. At this age, the hearing children will begin to exclude the deaf child, not because they feel any animosity, but because communication is sometimes difficult, and they don't want to take the extra time. They are intent on, for instance, playing the game, or getting their ideas across, and having to take the time to communicate with the deaf child slows them down. They don't intend to hurt feelings, or to be exclusive, but they are because that is where they are developmentally.
At about the age of 11, chidlren are beginning to develop a strong sense of identity, and are selecting among various potential selves and who they may become. This is where role models are extremely important. A deaf child in a mainstream school has no role models on which to base this exploration. All he/she sees in his/her environment are hearing people, and this is expecially true if he is the child of hearing parents. Therefore, his self concept begins to become distorted. He can't experiment with different identities based on the hearing people he is exposed to, because he knows on a deep level that he is not the same as the hearing people. He begins to become aware of his difference, even if he can't explain what he knows. It is at this age that deaf children in the mainstream often begin to try and prove that they are the same as their hearing peers, to deny that they are different in any way. This is also the point where he/she begins to internalize the concept of themselves as being different, and different is equated to wrong. They struggle to fit in based on what they are seeing in their environment. The deaf child who has interaction with other deaf peers, and role models in the form of deaf or hearing, signing teachers, has a completely different experience. He is able to esplore his potential selves based on realistic environmental feedback. His difference is not a concern, because there are others just like him. He develops self confidence and security in his abilities. He is provided with both the linguistic experience and the social experience that leads to psychosocial adjustment. He is free to learn without constant stuggle. He can concentrate on educational tasks because his psychosocial and emotional needs are being tended to. He freely communciates ideas and asks questions, and receives feedback and answers in the way he can understand them. He is able to complete his developmental task on time, and is free to grow as a person.
When chidlren enter adolescence, the task of finding one's identity continues. At this time, peer influence becomes of the utmost importance. Differences become the reason for exclusion, and children explore what it is they find acceptable and good, and what they don't. Acceptance by others in crucial. Most deaf children who have been mainstreamed arrive at this point with many coping skills they have developed in the earlier years to protect themselves from the pain of exlusion, or to make their differences appear minimal. While these coping skills might lead one to believe that the child is well adjusted and coping well, the skills they have developed are often unhealthy. They have not managed to develop an identity of their own, and their identity is dependent upon what they believe that others will accept. They are fractured. This is the point that you will often hear deaf adults speak of as feeling as if they were stuck in between 2 worlds. Girls may develop eating disorders in an attempt to gain some control over their life, because they feel so out of control in other areas. Boys may develop high risk behaviors. These chidlren appear to be happy and well adjusted on the surface, but are miserable underneath. They don't let their misery show, because that would only point out their difference to a greater degree, lead to more ostacism by hearing peers, and more misery. These students often become overachievers. They do well academically by memorizing everything they need to do to keep thier grades up. They work themselves into a frazzel trying to keep up. The hearing teachers say, "Look how well he/she is doing. They make great grades, they have lots of friends." The hearing parents say,"Look. My child is successful academically. They have lots of hearing friends. They have assimialted well into the hearing world. Mainstreaming has been a success. It's a wonderful thing."
When we look below the surface, however, we see an adolescent who has not been able to complete many of the developmental tasks of childhood because they have not been given the intitial tool to do so: language. Without being able to move through the earlier developemental stages, they are prevented from moving through the later stages. They become masters of pretense. They pretend to be okay, because that is what the believe is expected of them. They build their identity on what others tell them they should be, rather than what they have learned they should be. They have no sense of self, and they have not been provided with the skills and the life epxerience that allows them to develop a healthy sense of self.
This continues a pattern that lasts a lifetime unless these children find an environment that allows them to go back and adress what they did not get in their early years. This can be best illustrated by the number of deaf adults who relate stories of finding the deaf community in eaqrly adulthood, and for the first time in their lives, feeling that they have, at long last, found their place.
School environments cannot attend to the educational needs of children when their psychosocial needs are not being met. Children cannot learn academic material inthe most efficient and prodcuctive way if they are stuggling with these other issues. Without language, not only are their educational needs not being attended to, but their developmental needs ar not being attended to. We have to see our children as whole and complete beings. One thing affects another. A deficit in one area will impact all other areas. We have to create environments where learning happens as it was meant to happen: naturally. In order to do that, we have to have an environment where chidlren are free to explore all possibilities based on examples they can relate to.
This is getting rather long, and although I could go on and on, I will stop here. I hope I have made my position clear, but if you have any questions, please feel free to ask, and I will do my best to answer them.