I agree with both Barb_D and albertino. As Barb_D noted, the test presents you with sentences that do not occur in real life. Since a class is a group of students and that is well understood, "nobody" says "a class of students". Instead:
1. The students attend the various (not "different") performances of their schoolmates.You are right, and your son's teacher is wrong. However, for purposes of his grade it is probably best that he pretend to agree with the teacher. (Maybe he will get a better teacher next year, or maybe you can do the teaching, since you seem to know more than his teacher.)
2. The class attends the various performances of the students.
You say that you are running out of the better teachers there because they are running out on you. Where do they go? (What countries?) Why do they leave? (Higher pay elsewhere?)