I wonder what some parents would do if their young non-christian children were 'forced' to sit at the same dinner table as a gay/christian girl/boy. Would they congregate outside the school and protest it was against their protected religious rights?
It would seem to me the same scenario as the parents who are protesting outside schools that their children should not be taught that it is normal and natural to be homosexual.
You can say our legal system will sort it out. But that is the significance of the Charlie Hebdo killings. An ideology that refused to accept the law was able to create enough fear that they brought in de facto blasphemy laws. So even though you are allowed to criticise an ideology you better not criticise a specific one, on fear of death.
A head might think of teaching lgbt equality or forcing some children to allow a christian girl to eat with them. But if some parents cause enough fear the school will back down because they will be afraid of the consequences. The parents certainly wont be afraid of the law.
Multiculturalism has failed because some cultures refuse equality/integration, want their beliefs to be dominant over the hosts and will fight to make that happen. RIP London.