Yeah, people being dismissive of bullying as being harmful hasn't actually been the main issue for a long time. The actual problems with fighting bullying are twofold:
Mostly it's insidious. For every time something extremely blatant is done like hurling a book hard enough to get a concussion, a hundred other incidents occur that are small and seem innocuous to outsiders. Shoulder-checking every day in the hall. Obviously talking to a friend about the target and laughing derisively. Mockery. Rumors. How exactly are teachers supposed to address these things? Suspend the kid for bumping into someone in the hallway? Punish them for talking to their friends? Try to track down the source of a rumor? Teachers have a million duties piled on them, and know a losing battle when they see one.
Even if the teacher does try to do something, it may or may not have any affect. We're long past the age when most parents believe teachers over their own children. A huge percentage of the kids who act like shitheads have parents who believe their kids can do no wrong and believe their children when they say things like "I didn't even do anything" or "Mr. So-and-so just has it in for me." If those parents are also in the least bit powerful or influential at all, it's a recipe for the school being too scared to do anything. The weird kid who wears trenchcoats and has absentee parents might get expelled for a single threat (gotta try to prevent any more Columbines), but the kid who actively terrorizes people might not even get a talking to if his dad owns the local dealership and donated a bunch of money for the new football scoreboard. But hey, at least this one is educational--it's the way the world works in general, so might as well learn it early.
Realistically, it doesn't even take shitty teachers to turn a blind eye to bullying. If a teacher knows nothing's going to come of escalating something (or worse, there could retaliation or setbacks for them) they might look the other way even if they'd rather not. And just get a little closer to the point they decide that teaching isn't for them. This issue is one of many reasons that close to half of teachers stay in the profession less than 5 years, and that nearly 10% quit before the end of their first year.