It's because they're not the badass cops we see in the movies.
They act more like the classroom bullies we see in movies.
Bullies don't pick on everyone equally. They rarely target the popular kids.
They go after the kids who are already at a disadvantage because of how they look or how poor they are.
So when someone steps up and matches their toxic energy, they lose their sheet and don't know how to respond.
Civilians think they're getting John McClane, but all they get is Farkus from A Christmas Story.
No one would be surprised if a stat suggested most school bullies grew up to be cops.