Water. Dampen your hair, then comb the bun as you wish, & it will likely stay put better, as it dries. That said, a bun is not the best hairdo for a young face. Boys like loose hair, & it's more flattering on most women, too.
Sorry didn't mean to click hate it. Anyways, I where a bun quite frequently. When it looks like that I just pin all the sides down so it's more round and closer to my scalp. It works most of the time.