As I continue to observe student teachers in their professional semester of college, I can't say enough that they need to understand that they aren't in the classroom to be friends with their students. In my own experience, I was only two years older than some of the students in the class, and I had no idea what I was doing. None of the one class I had in classroom management sank in.
That being said, if you are the cooperating teacher, you're going to have to help them understand that they can be too sing-songy and that sometimes they must be firm. Students need to have a healthy respect for the adult in the room, and sometimes that means getting tough (not mean - tough). Sometimes that means the instructions have to be clear and direct. The teacher is not in the room to babysit (You don't get paid enough to be a babysitter anyway.), and the teacher is not in the room to be the entertainer (even though there is a healthy amount of entertaining that's necessary): the teacher is in the room to get the job done. Sometimes that means people are going to be upset, especially after making weak choices. In fact, the teacher may come out on the other side with even more respect from the students.
Leave the room. Sink or swim (but don't let them drown).
OK, mentors. You have to leave the room! Student teachers probably don't even want you there most of the time. You need to be available at the beginning so you can do some coaching. Remember the student teacher has not done this job before, and you need to impart your wisdom to them. However, at some point, it's sink or swim. Just like they don't want their students to melt down, you don't want them to lose complete control either, but you also don't want them to continue to pursue this career if they're not going to do it well. The way you find that out is to let them fly solo.
No, you don't want them to crash, but if they make mistakes, they need to learn from them, pick themselves up, and keep going (with alterations). If they don't pick themselves up, maybe this is not the career for them. It's too bad they didn't learn that before their final semester!
Be honest with the student teacher.
You may be just like them: you don't like to deliver bad news or consequences to your protege any more than they want to upset the students. Hopefully, they can take the truth, if delivered with love. Remember though, they aren't seasoned veterans: they are beginners. Don't expect them to have all the answers and be perfect teachers. That's why they are in your class, and you'll need to help them achieve the next level from wherever they are.
If they are really good, that means you need to still take them just a little bit higher. They need to feel what it's like to improve. That might mean you have to point out when they have climbed the next short rung of the ladder. Student teachers can be pretty rough on themselves and not recognize when they have done better. At the same time, that means you also need to show them how trying the same things over and over without change will always get them to the same place. They need to know that they can take risks and that they can be vulnerable with you. Remember, we aren't in their lives to make them miserable with heavy burdens: instead, we must help them carry and keep them moving forward.