Credible threats of gang violence on a regular basis. School-wide lockdowns with some regularity due to threats of violence, or violence that has actually occurred. Admin with extremely limited options to deal with any of this, and having to triage their discipline pretty hard, plus having little to no support from home to back any of it up (this is why the stuff in the first paragraph happens constantly—no time or community/parental willpower to deal with it, given the rest of what's going on). Watching the kids suffering from the same shit day after day, plus all the usually-awful stuff they have going on at home, and not being able to do much about it. Seeing kids die or end up in the hospital with alarming regularity. Distressingly young kids plainly high or drunk in class.
Not many people can do that without burning out or giving up and half-assing everything, very quickly.