At the risk of sounding like one of the children in question...yeah, but why?
I can imagine, and indeed have experience of, classes discussing such subjects appropriately.
The inclusion of the content in both the case of pornography and these cartoons would be gratuitous. There's no need for it to enable a productive discussion.
Kids no doubt can, will and have found these cartoons and pornography for themselves. I'm not arguing for any sort of bans, just I'm not sure teachers should be showing this kind of content in schools when it's not necessary.
If the teacher had just mentioned the cartoons and the attack that would be acceptable do you think?
His Lesson was about freedom of speech which implies freedom of thought and by implication freedom of expression. If he felt curbed in presenting his lesson in any way then the lesson would be meaningless surely?
The person who beheaded this teacher had no understanding of or appreciation for freedom of any kind. Let alone freedom of thought or freedom of expression.