Thanks folks. I'm pretty rubbish at making tea and sandwiches never. I'm pretty good at teaching quadratic equations to teenage boys tho!
I decided to just disengage. The reasoning was that young, female, childless teachers like girls (because they sit nicely and read books) rather than boys (who are too boisterous for them to handle), and have no idea about family life (presumably because we all magically appeared, fully grown at 18, with no family and don't know any families / children outside work). There was so much wrong with that I didn't even know where to start!
I'm finding it increasingly hard to just ignore the sexism, but maybe that's because I'm facing an onslaught at the moment. DSis who keeps telling me how I'll feel "when" I'm a mum (I have no plans to be in the near future), a range of people expressing shock / concern / pity because I ended my relationship with fuckwit ex-fiancé (literally in the same breath "you seem so much happier" and "don't worry you'll be married with kids one day"), colleague making sexist "jokes" then getting annoyed when I point out that casual sexism isn't funny.
Fortunately it's nearly the end of term, and I'm having a large glass of Baileys when I get in tonight, watching the space shuttle dock with ISS and admiring my beautiful Christmas tree in my own (rented) house.