Steven not knowing about schools was an oversight.
Steven takes after his father as well as his mother, and great as Greg is, he likes to take the path of least resistance a lot. Gem stuff is complicated and stressful, and Steven is trying his best to cope with it as it is.
Pearl spent most of her time on earth devoted purely to Rose. What exposure she had to human culture was most likely incidental, or at Rose's behest. Ballet was probably a part of gem culture as well, since they all have dancing. Driving was probably picked up at some point while raising Steven, since Greg mentioned that long rides helped calm him down.
Fusion doesn't require perfect sync, it requires a close, intimate connection with the other person, being vulnerable and open with someone else. Dancing is a means to an end, and dancing without that connection wouldn't work.
We don't know that no humans know about gem stuff, just that the beach city residents are pretty chill about it.
Gem technology is extremely complicated and nearly all of it has biometric requirements in order to function and a lot of it seems like pure magic. Reverse engineering it would have been impossible for most of the time, and whose to say that earth's military hasn't got gem weapons?
Steven is afraid of sex, especially the sexual attention of a male, and that fear turns to anger.