I think it's more the fantasy that a girl who is of tender years, and should look demure in her uniform, actuially sends out signals that she is voraciously sexually active, and that any man in her company is guarenteed some serious enjoyment.
This juxtaposition is expanded by the 'fancy dress' version where a more mature woman wears the uniform, but with obvious erotic signals - very short skirt, open blouse, suspenders, combined with the 'schoolgirl' aspects - hair in bunches, freckles added, and so on.
I never had the chance to fancy girls at school, I went to an all-boys' school, and I was seventeen before I spoke to a girl who was not a friend of one of my sisters.