Can a teacher blame the whole clroom because of one person? Can he actually make the rest of the cl pay?

Hi, well...I was wondering about this for a while, my sister is in middle school, sometimes she gets out of school very late because the teacher grounded the whole cl because of one child that didn't behave well or a group of children. I believe this is absurd, really, I mean my sister is one of the top students in her cl, very hard working and quiet, so shy and quiet that sometimes teachers tell my parents she needs to talk a little bit more, and yet she gets grounded along with the other children because of one or 2 children! i don't think this is fair at all! This summer most of the other cles are going to different places including a baseball game, except my sister's cl for misbehaviour... and I was all like WHAT??!!! ...Ok the staying up late after school is one thing...but still wrong, why are the innocent children being punished with detentions, extra homework and no lunch time? At first I thought...well maybe they have to learn to be a group and work together to solve these problems and I thought they could work it out, but it's almost end of the course and those problematic children are not behaving well at all! i don't think it's my sister's fault that t he parents of those problematic children won't do anything to correct them. Anyways, I was wondering if there's anything the parents can do, besides transfering her from schools because that's what we are going to do next, but we are hesitating since she was her elementary friends there too... Any ideas? Thank you guys...