Oh, please don't stuff your bra. You don't need a large chest to be beautiful, you're beautiful just the way you are. If you can't fit in a AA cup, then maybe you just haven't started to develop yet. Everybody grows at different rates, so just wait. I'm a 34A but I know that someday I'll grow. The average woman stops growing in her 20's, so please be patient.
People will find out if you're stuffing. Think of it this way - wouldn't you find it weird if all of the sudden someone went from totally flat to a B cup in one day?
Honestly, there are things that you can use but i definitely don't recommend them.
Bra inserts fill out your chest, you can just slip them into the sides of your bra, but be sure not to use a sports bra or a triangle bra! They're available for $48 at herlook.com. They're a bit pricey, another reason not to stuff.
You can also buy a padded bra pretty much anywhere that sells bras, but again, please don't. You don't need a full chest, if you're as flat as an AA, then that's how you're supposed to be