People are perfectly free to not let their children go on any school trip for whatever reason they see fit, and a belief that Islamic cultures are generally speaking in relative terms more sexist than others is IMO a fairly reasonable one and if they don't want their children to have to cover their heads or for them to see other women, like their teacher, have to do it, well that is entirely their business.
They don't need to ask the permission of the self-appointed culture police on this thread, or to spend hours justifying why British culture is just as bad in every single way to satisfy their silly, navel-gazing cultural relativism. Islam is a religion, not a race. Plenty of people aren't keen on religion at all. Plenty of Muslim families opt their children out of visits to churches, sex education etc. I'm sure that's entirely different though, isn't it?
There are a few posters here who appear to like nastily mocking people's intellect, sneering at people and calling them racists and bigots rather more than they like putting forward a reasoned argument. Here's one for you, from an Iranian feminist:
maryamnamazie.com/articles/racism_cultural_rel.html
Let us be clear about what cultural relativism is. It is a profoundly racist phenomenon, which values and respects all cultural and religious practices, irrespective of their consequences for women. It asserts that the rights of people, women and girls are relative to where they are born, "their" cultures and religions. There is no right or wrong according to cultural relativists. As a result, cultural relativism supports and maintains sexual apartheid and violence against women in Islam-stricken societies like Iran because it is "their culture and religion" and it creates ghettoized, regressive "minority" communities in the West where women and girls continue to face apartheid and Islamic laws and customs.