Why didn't they escape?

Hmmm, that's actually a really good point. I have never thought about that before. So many crazy things were happening at that time in the movie that my attention was always focused on something else. So, my answer, unfortunately, is "I don't know". Hopefully other people will post their thoughts.

Then they would've been able to stop the "riot," and the film would've lost its so-called message.

remember when they (the cops) were in the parking lot before going into the school? Doberman said something about no guns and one put his in the trunk of the car

My Signature...

As somebody noted, the movie makes a point of showing the cops putting their guns in the trunk of their patrol car before going into the school. However, it sure seems like they could have broken a window on one of the doors and gotten out. Even then, there would have been risk, if you can picture a chained door without glass, it would have taken some contortion to get through it and it would have left the first person out vulnerable, without weapons, and surrounded by rioters.... that doesn't feel like a very safe place to be.