I don't get involved with the IEP process, so I can't address how that's handled.
In the elementary schools, I've noticed that the interpreter is more of a teacher's aid in that do additional duties beyond the scope of interpreter. This is especially true with multi-handicapped mainstreamed deaf kids. Honestly, I think elementary kids would be better served by having signing teachers rather than terps. I don't think the little ones understand the role of the interpreter.
Starting with middle school, I notice the terps become more like terp/tutors.
My personal opinion is the interpreters shouldn't be left to watch high school students when the teacher leaves the room. Classroom management should be handled the same way for all classes, whether a deaf student is present or not. I'm OK with the terp staying in the room if it's for communication purposes only for the deaf student. I'm not OK with the terp being left to keep an eye on all the students, unless it's a true emergency. It shouldn't be an expected routine duty.
I have the luxury of being a substitute public school terp. I tell the principal or sub coordinator up front that I will be interpreting only, nothing else. So far, they've been fine with that. I do know full-time school terps who have had additional duties, including administrative tasks, bus driving, diaper changing, and medication dispensing.