So here's a thing on this Des Moines shooting, not necessarily connected to the shooting itself.
Videos show the shooter was involved in a dustup of some kind last month, wherein he was waving a Confederate flag at some kind of demonstration, at the Urbanville High School. The cops told him to leave, this Confederate flag-waving being in violation of school policy. One cop is shown telling him that if he waves a Confederate flag in front of a gathering of African-American citizens, he has to expect some sort of reaction.
That led to the shooter going off about the cops violating his civil rights, etc etc and yadda yadda. The rest is history.
OK ... there is a big difference in a school policy that prohibits students from wearing or displaying items likely to cause a breach of the peace, and a 'crime.' It can easily become a First Amendment 'issue,' because it is a First Amendment 'issue,' but that is between the students, their parents, their lawyers, and the school district.
Clearly, an adult waving a Confederate flag at a demonstration, on school property or not, is also a First Amendment thing. Same thing for when he was in the stands at some athletic event or another, waving his flag.
I don't think as a cop I would have told him to vacate the premises, school policy or not. Why get dragged into a civil rights battle you're going to lose?
The way to deal with those things is ... when the school official comes whining up to you - as a cop - about how some character is violating school policy and causing problems ... you tell the school official to tell the problem person to leave. That person refuses to leave, and now you have criminal trespass, which is a clear legal thing. Whatever led to the person being told by the school official to leave (as opposed to the cops doing that) is now between the school and the person. And his lawyers.
Cops should not be enforcing school policy. School policy can be very iffy stuff legally, and especially criminally legally. Cops should be enforcing La Ley, not school policy.
That's no excuse for this neo-Confederate POS shooting the cops, of course, but it's an underlying 'issue'.