There are some rotten incentives at work here, as well as constraints that aren’t obvious from a parent or student’s point of view.
For example, CA schools have to publish statistics on suspensions and expulsions. So there’s an incentive for administrators to minimize them. In practice, this means that expelling a student (short of some extreme situations) is a lengthy process of ass-covering. Even when administrators are doing the right thing, from the outside it can look like nothing is being done. Think HR putting you on a PIP.
Meanwhile, the “right thing” isn’t always so obvious. The “violent and disruptive” student is also a child with a right to an education. And for what it’s worth, usually a child in crisis. For school staff, your role as an adult is to teach the child to participate in society with whatever limited influence you have. As a parent or classmate, of course, you have no reason to give a shit about some asshole kid, but the teacher has to.
And then, what does “violent and disruptive” actually mean? How much violence? No tolerance? What about a bullying victim who sticks up for themselves? Playground scuffle? At what point does the dial turn from teaching a child not to hit, to teaching a child that they are bad and do not belong? What about non (physically) violent bullying? What about children who are disruptive, but not violent (surely including a lot of those posting here about how their ADHD was misunderstood)?
Sometimes expulsion is the answer, even keeping in mind that every student expelled before 16 is just going to school someplace else. But the problems are more complex than people often realize.