if you are younger than 20 it's natural, I'm 21 and it still happening to me not as often as when I was younger but it happens.

I believe it is normal, the thing has a mind of its own. A lot of times during high school, when I sleep in class (which was very often) and my upper body was bent over sleeping on the table, I'd catch a boner for some reason in that position. I'd wake up to the bell and going, "shoot, if I stand up to go to the next class...".
Maybe I said too much =/

Eer so yea, don't worry about it so much.

Ya, I have the same problem, but I'm only 16. Sometimes on the bus, I will randomly get a boner and I'm afraid to stand up when the bus stops, I always stay sitting down a little longer than I need to and take a nice deep breath.

Adjust yourself, put the backpack over top of you and just straighten er' out.

And this happens all the time...

just wrap it in an upward direction (less noticable that way) and walk away