Is this too much for a 12 year old?

Well I think it depends if she's handling the schedule well. If she seems tired and worn out, it might too much. If she seems to be doing fine, then I'd keep her in it. However, horseback riding, gymnastics, ballet and choir seems like a bit much. But if she likes it right now then there's no problem. If she wants more free time in a few years I would let her cut back on all the activities.