Well I'm 12 and I have a bunch of friends my age that have double peircings on the lower part of their ear. It really depends where you want your double peircing. If it's on the cartlige, then I'd wait a bit and think about it because its generally painful. But it's the same as my friends, and at the lower area (anywhere non-cartlige) I would feel free. What harm is a peircing going to do to a growing ear? Plus, if it's not on the ear, but anywhere else on your body, I would definately pause to think about this. 1 because they can be very painful, and 2 bacause of your school's policy (would you be allowed to do this?).
Hope this helps ^.^