I haven't wrestled with exactly this, but I feel your dilemma!
I would guess that if it's a K-6 with 48 students it doesn't have year groups which are full (unless it's filling from the bottom and doesn't actually have any children in the higher grades yet)? My first impulse would be to talk to someone at the school about how much you love it, express a wish to put Hanni in it from Sep 2013 at the latest, and be open about your wondering about whether Sep 2012 would be better for her. See how they react to that idea; see how they react to an idea that if you don't put her in this year but end up wishing you had, you and they might at that point have a conversation about whether she might be better to join in 1st grade in 2013. Have they met H? It may help the conversation if they have... If they aren't full and you establish good relations with them, I'm thinking, you might even be able to decide to switch her in part way through next year at a point when you're confident she's socially and emotionally ready for K; that might end up being ideal. (IME, children can arrive at any time during the first couple of years and slot in easily - stable friendships that are hard to break into come later.)
Congratulations on finding such a great option, but I'd definitely be trying to keep the bird in the hand until it flies off!
