We join Miss S in the middle of a fine rant denouncing sexism:
Miss S: ...The only difference between boys and girls is that males have an Y chromosone in place of the one of the female X chromsones. Everything that isn't on that chromosone is identical.
Pupil J: That's not true Miss. Boys have a meat-and-two-veg, while girls have a pineapple.
Miss S: A pineapple? What makes you call a woman's bits a pineapple?
Me: You know Miss, I'd rather not know.
Miss S: You're probably right Sir.
 I should note that Miss S has a degree in biochemisty and is better informed than me about the embryological differences caused by the expression of the sex associated genes, and has a good overview of the gender differential cultural pressures in modern society, but when dealing with a bunch of bolshy 13 year olds has to skate over the thornier subtleties of non-mathematical topics. As of Friday afternoon Miss S is also the most recently qualified teacher in the country. Yay!