Suppose you are a kid who's getting bullied by a larger kid. Every day the kid takes your stuff and messes with it or something. You (maybe) complain to the teacher, but really you're scared that if the teacher leaks that you complained, the kid will beat you.

So eventually you grow tired of it, you've worked out a bit, and now you beat the kid up. Now... you get suspended/expelled, instead of the bully. That's a zero-tolerance policy.

My wife's cousin was suspended for a few weeks for standing up to her bully. My in-laws and her family decided to reward her and she got two weeks of a fun family vacation. But the school was trying to punish her, and if her parents and grandparents weren't more sensible, perhaps they would have succeeded.