I did a research paper on this last semester and the information I got came down to that deaf children who attended deaf schools, who routinely teach below grade level, are strongly ASL based which uses English words with a different syntax and grammar than straight up English does.
Also deaf children who have had very little English learning (as in an all-ASL based learning) tend to read sentences in a linear manner than a hierarchial manner. (not all obviously, but those that have had little to no English instruction or very poor English instruction). For example:
The boy threw the ball, and broke the window. <-- original sentence
A boy. Ball broke window. <-- how a strongly ASL based student may read the sentence.
Surprisingly enough though that ASL-based students tend to have little trouble reading simple declarative sentences. The deaf students understand the words, but not clear on the order of them or why, as again, ASL has its own grammar and syntax apart from English.
I'm not saying that ASL instruction is bad as I am a firm believer that a deaf child needs a solid foundational language to which all other learning can take place on and grow.
Another fault within the public education system is that when they teach students to read, they teach them to read phonetically - IE - sounding out the word. To a deaf student this becomes a tedious and meaningless task, especially if they cannot hear in the first place.
We need to teach deaf children to read the way they experience the world - visually. Of course when you put pictures in place of words in a compound sentence, it tends to lose its meaning I think. For example: (using the same sentence as above using pictures)
Its the same sentence but pictorially it's interesting but it becomes confusing.
This is why I am in agreement with Shel90 and others about having a Bi-Bi approach to learning with deaf children whether they are in a deaf school or mainstreamed in a public school.