Interpreters sometimes do speak for the deaf person who's signing, but that all depends on the deaf individuals if they want to use their voice to ask a teacher a question, for all that means they may. I've done it during my high school years, when I raise my hand, and a teacher calls my name, I would voice my question, and the interpreter would know that she/he would not needed to voice the question for me.
If a teacher does not understand a deaf student's question the interpreter then will voice the question again for the deaf student. That's one reason why interpreters are there for the deaf, in case we need them.
When I was in public schools with no interpreters, it was more frustrated to understand the teacher by reading her lips the whole entire period, because sometimes the teacher intends to face the board and begins to talk, how can a deaf people reads the teacher's lips when her back is facing the deaf students face? (that was an oral method program)