Most teen guys get erections at embarrassing times, when it seems like there's absolutely no reason for it. You might, subconsciously, be thinking about something sexual. Or, it might just be your huge hormonal shifts that cause your genitals to be supersensitive. But don't worry.  Although it may seem like everyone will be able to tell, no one can really notice. 

Spontaneous erections happen a lot less once a guy finishes puberty, although adult men still get them every once in a while.