Back when I was in school, admittedly quite a long time ago, the school could discipline you for anything you did between when you left home in the morning and returned home after school. Or possibly went to work or some other location where you obviously weren’t “in transit” anymore.
Not saying I necessarily agree with this policy, but it certainly isn’t completely new.
It sounds to me like the Principal is trying to find a way to tell the students,
“Don’t go to down in a pack/mob because there is a strong tendency of young people in large groups to assume that they are individually invisible. They sometimes (often) turn off the training, moral restraint and common sense that their parents, community and the school system have worked to instill in them. They end up acting like idiots, fools or worse. Their ensuing behavior can cause embarrassment to themselves, their parents & the community. Sometimes it can lead to property damage, physical injury or even death.”
“I’d tell you what I really think, but the ACLU, the local hot-shot reporters and the school board legal adviser would have conniption-fit.”
“Have a nice day, get your homework done on time and, by-the-way, your dog called to say that he prefers Alpo or Kibbles n’ Bits to eating said homework.”