I agree with Hunty that the Muslims on this PTA committee seem to be extremely conservative.
I don't think that people and institutions should necessarily give religious concerns automatic priority. But if running an event in a certain way would mean that 50% of children and families wouldn't feel comfortable attending, then it is the wrong event to run.
So the big question is whether these religious conservatives really speak for the other Muslim parents, or whether they are trying to strong arm the PTA with their own particularly extreme views. The way to find out is to consult widely - not to come back with "respect our culture" bullshit.
I don't think the idea of having a disco (or indeed a quiz night) or whatever is OFFENSIVE to Muslims. What is offensive, and racist is the suggestion is if someone has bothered to volunteer and to go to PTA meetings, that their suggestions should be overridden 'because they are not English'
Equally it is lazy and racist to assume that these few people speak for all Muslims at the school.
It would be perfectly reasonable to do a survey, and if it turns out there are plenty of Muslims who would send their children, then go ahead andhold it, even if the religious conservatives wouldn't be able to attend. On the other hand if the feedback is that 50% wouldn't go then the PTA should probably try another idea.
When I was at school we never had events after school on Friday. It wasn't because Judaism was being given automatic priority, it was because a third of the kids were Jewish and it would have been rude and inconsiderate (and dumb) to hold events that would exclude 1/3 of the pupils.