I am new here...so sorry if I say something already mentioned. I know there are agencies that require the student's signature as well at college settings. I think the professor should be enough if a signature is required, but I have not had any problems with the students I worked with. Most of them are just happy the agency was able to send an interpreter.
As far as firing an interpreter, I believe there are a few Deaf clients that really do not understand the role of an interpreter. Please don't take this that I mean all. Many of the misunderstandings I have witnessed have come from students. While interpreting, I have been lucky to have knowledgeable Deaf adults about the role of the terp. For a few years, I worked mostly Deaf Services/Case Management and not much interpreting. While working in the Deaf Services field, I had many clients that were clueless about the interpreting process/terp role. Many of those clients felt they could "fire" any interpreter they wanted. They felt the interpreter had to respect them, but they did not have to give respect back. I had to set up several workshops on How to Use an Interpreter for my clients and hearing businesses as well. I had clients demand things outside the interpreter role, and then they got angry at that interpreter when they could not comply with something against their CPC's. I had several clients that would then try to bad mouth other interpreters. I explained the ADA specifies "qualified interpreter" and not favorite interpreter.
I have lived in several states. Most of the misunderstandings happen in states where the interpreting standards/knowledge of interpreters are not up to par with the rest of the country. Also, it is sad that there are interpreting agencies that are clueless on the interpreting business. These are agencies run by people who have not interpreted before or been "in the trenches". These so-called agencies will sometimes ignore interpreters and just side with Deaf clients and especially students (even hs age). It is hard enough to do your job, but even harder when a hs student dictates your job. As a Deaf advocate, I have seen this happen while advocating while someone else was interpreting. Some agencies, who don't have someone with previous interpreting experience in management, see dollar signs and nothing else. My heart did go out to these interpreters who went through this.
I am happy to say I switched back to interpreting from Deaf Services. It is something I enjoy much more. Sorry for the book.
I suppose I should do that 
