Alright, let's take this from the top. So, the police received threats that some students were gonna go on a rampage and possibly hurt some people. There were possible gang connections involved. The police responded to that threat by closing down all public transportation and not letting students go home, and just generally being super authoritarian and threatening. They showed up in full riot gear, kicked everyone off the buses, closed the subways, etc. In other words, they intentionally created a massive crowd of pissed-off teenagers, knowing that there was a possibility that some of them were planning to riot.
This means that they a) put kids who wanted nothing to do with the situation at risk by stranding right in the the center of things, and b) unintentionally helped the riot to form by congregating large groups of pissed off people into small areas while wearing full riot gear. I'm making the argument that these actions were dumb as fuck. I'm also saying that, because they made bad decisions that helped to make everything worse, they're partially culpable for all of the chaos. Finally, I'm musing that perhaps this is more of an unfortunate series of idiots being dumb (and to be clear, I include the assholes robbing the CVS etc. under the flag of 'idiots') than the standard 'rioters ruining everything' situation.