A civics teacher is in trouble for showing an incredibly graphic video of a murder and dismemberment to his class. The guy warned the students, who seemed to be junior and senior level in high school, took a survey of who did and didn't want to see it, and offered an out for those who didn't. I think his main mistake was not notifying the parents, but it's not a huge mistake, IMO. We let teens that old drive cars, they're definitely old enough to make a judgment about something like this.
That said, during one of my cultural anthropology courses when I was, as I recall, a sophomore in college, we got shown a film which graphically depicted the ritual slaughter and butchering of a pig. Lots of warning was given, and as I recall some students didn't actually attend. Even then, at least two got so upset they had to leave (in tears, naturally), and the professor was visibly embarrassed by it all. Which is to say being warned something is horrible is not the same thing as actually witnessing something horrible.
It's a pretty questionable judgement call, I think. But if the guy is otherwise an upstanding, high-performing, and popular teacher, I'm not sure he should face any serious discipline over the incident.