Honestly, even IRL much of the time, someone saying "go kill yourself" is basically just being a troll. And a complete jerk, but mostly a troll. Usually that sort is nasty towards LOTS of people. Until someone snaps and punches them or something.
Even so, it's really not acceptable behavior. If someone said something like that to me IRL, I'd probably make a point of dumping my drink on their head (and it's always soda, which gets STICKY as it dries out), followed by a very detailed explanation of why it'd be fantastically stupid to try to get physical with me in retaliation. Unless I"m in a decent mood, at which point I'd probably just stand there, drinking my soda and getting amusement out of watching them make an ass of themselves. Particularly since the longer they go without getting a response, the dumber they tend to look to most people. I used to do that in highschool. It could be pretty funny to watch them slowly wind down after 5 minutes of childish rambling.
And of course, in school, well.... goes with the territory. There's always bullies, they're always dimwitted, and they cant seem to be able to find other ways of venting that DONT involve being amazingly stupid asshats. As for the teachers... yeah, some of them need to PAY ATTENTION, but they also have tons of duties, and lots and lots of students that they need to keep an eye on... it can be very difficult to do that, particularly as many people just arent very perceptive to begin with. If you want a teacher to help, go to them directly. Or even go to other members of the faculty. In many cases, there's always a few people that are definitely willing to help.