"You lot" is a rather rude way of addressing a group of people, especially by a person in authority. In this case, it's a teacher speaking to a class of noisy students.

A "lot" is simply referring to a group. The teacher was saying the next one in the group (lot) who made noise would have to stay after class. Judging by their choice of words, it sounds like the teacher was unhappy when they said this.

"Lot" in this context means a group. i.e. "The next one of you lot (of people) who makes a sound..." In this case"lot" could be replaced with guys/people/boys/girls and still make sense.