It's becomes a problem when a lesson can't get through. I remember in high school we'd miss an entire lesson and get left behind because most of the class was loud and disrespectful. And those teachers tried their hardest to get the lesson done.
I can't even recall a time even with a sub where our classes got sidetracked like that. Definitely another fairly intangible benefit about going to a good school in a good neighborhood, I guess, when the usual focus on troubled schools and troubled kids is on grades or home life.