When I was in school boy and girls both had 8 periods a day.
The first ‘dirty’ joke I ever heard (and trust me I was waaaay too young to understand it, at the time) was:
Why was the little red schoolhouse red?
You’d be red too if you have 8 periods a day. (Cymbal crash)
I was the only girl in a family with several boy cousins (all older). I begged them to let me stay on the sleeping porch one summer night when we were visiting. I must have been about 7. I never forgot the ‘joke’, but it was several years before I understood it.