They're usually M3/M4 students, but some M2 students can do it too. I had a subpar experience with a M2 student interviewer at one school--ze was not very friendly. But, my friends had great experiences interviewing with different student interviewers elsewhere. The person I interviewed with told me they had to apply for the position and were voted on by classmates to take on this responsibility. So yes, I would assume they are extensively trained in this.