She might not want to ride to school if going up is getting there instead of comming back home.
weight weenieness definitely makes much more of an impact with 110lb girls than it does with overweight guys. I found the Giant FCR3 was way better/faster/easier to climb than all the bikes $100 less than it (trek, specialized, kona).
My wife and 12 year old both like dancing on the pedals up hills more than spinning, so I'd focus more on weight than gearing for them. They're still at the stage where they get off and walk big hills though.