Unless I'm mistaken, we never actually really see her boobs - just a semi-side partial view with her wearing a bra through the telescope - which could be a stuffed bra for all we know, or she's a ladyboy. As for the rubbing up and down on the teddy bear, that doesn't mean Mia's a girl as for all we know there's a hole in the teddy that a guy/ladyboy could use for stimulation.
Mia is an obvious trap, which also doesn't discount that fact. Consider this - the mother is a strict religious control type - it could be that she really, really wanted a daughter because she views "men" as all evil, corrupt mini-devils and thus need to be controlled/dominated to keep them god-fearing and purge them of Satan's influences & corruption (like lust, masturbation, etc)... and when the 'son' was born couldn't accept it. So dressed the child as a boy, taught the boy to act, speak like, dress, etc as a girl... which is one reason why Mia is NOT allowed to date or have friends over - dating/friends-over/sleepovers elsewhere runs the risk of the truth being found out. Obviously the religious aspect plays heavily on this as well... so could just be religious reason's.
As I said, it's an obvious potential trap to me, but being obvious doesn't mean it should be discounted.
And the other thing is... are we talking trap as in boy who dresses as a girl, or a possible lady-boy... who has boobs but also a penis? Mia could still fit that role. Or reverse-trap - a girl dressing as a guy (although this would be harder to pull off, but is possible).
IMO the two obvious/stereotypical 'traps' are Mia and the goth girl. They both are the stand-outs for being a 'trap'... which makes me think that if the creator is trying to be tricky, it'll be neither. Say maybe the principle or the prim-proper hall monitor being a lady-boy (since we have seen boobs). If we go for the ladyboy trap, then the bank teller is also the stereotypical option for that.
I doubt it'll be the Aunt, mother or sister, since I suspect the creator doesn't want to spoil the 'trap' right away and wants to leave people guessing.. and both mum & sister are too close to you and would have been known prior.