Nbd or Non-Intentional boner disoder. Usually happens to boys around the age of 10 to 16. This is an uncontrollable event and completely normal. Once a boy becomes a "man" he will no longer get Nbd's. Unforetunately some people will experience this disoder for a longer time than others or possibly forever. That is why you don't want to use viagra. At a young age.
Jill: "Hey jack"
Jill: "What's in you're pants?"
Jack: "Oh, that? Just my enormously large cock I can't hide"
Jill: "You mean you have Nbd"
Jack: "I had the hardest Nbd last period"
John: "I know my pants are so tight it hurts"
Jack: "Man, Nbd's are annoying"
John: "I know mine was harder than a goats butt in a jalepeno pepper patch"