I agree with a PP that the line as to what is school's responsibility to teach and what is the parents' is getting blurred. While some is lazy parents, some of it is the fault of schools (not saying your school, OP, just some I know of) that insist they know better than parents on some issues, which leads parents to assume the school will take over duties to go with those rights. For example, several primary schools I know started monitoring lunch boxes for "healthy eating". There was a big hullaballoo on another forum when this came out, with parents asserting that making sure their kids are fed properly is their job, but the schools seem to think otherwise. The biggest problem was that the schools weren't doing it properly - one child was told her humous and vegetable sticks was unhealthy and not to bring it again! Schools now assert the right to control when children should be in school (even if a parent wants to take them out for a once-in-a-lifetime learning opportunity), what they eat, how much exercise they should be doing, when they see the dentist, as well of course as what they wear and so on. It's not surprising that some parents assume this means the school basically takes over parenting in other ways too.
I think, "It's not on the curriculum, I'm afraid." is perfectly polite, OP. I'd perhaps with persistant ask-ers run down what DOES need including on the curriculum, and how it is therefore absolutely full with no space for added extras.