ANDY'S ANSWER: Ah, those inappropriate erections! They are the source of embarrassment for a lot of boys. I have talked about this very question earlier here:
If you're asking what he can do to make it go down quickly if there is nothing to hide it like a book or a jacket, he can either reach into his pockets and try to reposition his penis into a less obvious position, or imagine someone giving his penis a smack. I know that would deflate me pretty fast.