The school did filter the website to some extent. But they aren't smart enough to know how to do it right. It is their right to do so since they own the equipment. Chris had already noted that they showed a lynching video in class that included a mutilated body. And the Corcoran article admits that F 911 was shown in a class. Last week a teacher's husband showed video from his homeland in Romania of atrocities by the communists. So obviously there's only some violence (by Muslims maybe?) that's not allowed. Or maybe it's the presenter that's the problem.
I think, as I mentioned above, it's not the links to Islamic beheadings that gives Principal Stapelfeld the willies, it's the stuff below that goes into what's going on in HHS.
The presenter is indeed a problem. A liberal can say things a conservative can't, like Kerry can say, "I will kill terrorists" yet a general cannot. There are things liberals can't stand to hear, as well.
From what I've read recently about Super Berman and the HHS, he's all over the web, he really does not want conservative attention drawn to "his" school and "his" favorite teachers, who are teaching a quite liberal agenda.
Remember when the president of Harvard U said that he felt women and men have different natural abilities? There was at least one liberal hearing these mere words who had symptoms of dizziness and nausea and had to stumble out of the lecture hall in shock, or so she reported. Dissent is not tolerated, at a visceral level, by those who would teach us to be tolerant of other ideas.
Saying that the website on the poster links to violence is a convenient way to suppress its dissemination, in a way that does not lead to more questions.
I think there may be something in what you mention about violence by Muslims not being acceptable you will find few people who even remember who it was that attacked the WTC the first time.