I don't know the background to this. If the teacher was using the cartoon amongst other items of material to have a sensible debate as to what could be acceptably described as "free speech", and how religious people should respond if they feel their beliefs are being disrespected, then to me it would seem a reasonable debate to be had.
Unfortunately, in the current situation where followers of Islam feel they are being unfairly marginalised and caricatured (and, in my view, there is some justification for them feeling this), it was unwise to use material that might be perceived as deliberately chosen to be offensive.
However, as it is said this teacher is now receiving police protection, I think that is an absolutely abhorrent situation. I do not believe that ridiculing somebody's beliefs is acceptable but I do think people should have the right to debate the rights and wrongs of all religions without being threatened and intimidated.