About prefects and Head Boys/Girls?

According to link Head Boys and Girls are chosen from all the seventh years. Perhaps it's easier to be chosen if you were a Prefect before, or maybe it was just a coincidence that all the Head Boys and Girls we know about were Prefects?

I'm pretty sure Remus was still a prefect. You're right, usually it's 2 prefects that become head girl and head boy. So I'm just guessing here, but I think James wasn't mature enough to be a prefect in their 5th year, and Remus obviously was. By their 7th year, he was, and James was probably more of a leader than Remus. Sorry, I'm just guessing here, someone else might give you a better answer.

Maybe this is just me not having read the books properly (shame on me if that's the case!) but where does it say that James becomes Head Boy? For the life of me I can't recall having ever read that.