JaneJudge the representation of the human face in Islam is forbidden because they consider that it may lead to idol worship - worshipping the image and not the person behind it.
That is the first reason why this cartoon was offensive to Muslims in this classroom. Secondly reason it was offensive was because the Islamic fath has very strong views on basphemy, far stronger than any other faith, so this cartoon is offensive on two grounds as it also represented Mohamed, and in an insulting way..
This is not an event where there are two sides right or wrong. This is an event where both sides are in the wrong. A teacher who shows a bit of paper in a class, where he knows it will outrage the religious beliefs of many of those present is in the wrong. I do not know what the topic under discussion in this classroom was, but tolerance is about respecting people's beliefs where they are not harmful and condemning them where they are.
For exaample, I would always respect a muslims desire not to see representations of their prophet's face in a drawing or cartoon, while condemning the persecution of Christians in Pakistan, a muslim counry, under their murderous blasphemy laws.
I think this teacher was stupid and disrespectful of his pupils. I think the demonstrations were completely unwarranted, and as always nowadays, within an hour or so, taken over by extremists with no involvement with this school.
A delegation of the parents, as there were so many, should have contacted the school and spoken to the Head teacher about this issue. There was no suggestion that the head Teacher was unsympathetic or did not take the subject seriously.