When I was in high school, nobody in gym class work a jock...except some of the jocks, the actual, sports-playing athletes. I guess they wore them for practice and games and just thought it was natural to wear it for gym class. I would sometimes try to time walking back into the locker room so I could catch one particularly cute athlete changing out of his gym clothes and jock.
So jocks always seems a bit mysterious to me. Many of the fittest, most buff guys wore them. That kind of made jocks more erotic. I always wanted to get one but never bothered. It's still fun when they crop up in gay porn though .