I would definitely stick with the school one until she figures out whether she really wants to play the french horn! I was in band for one year (totally unmusical, I am) in 7th grade, and wanted to play the trumpet (because we had one)but so did everybody else and I ended up with the french horn. The one the school had looked like the one Winchester played in M*A*S*H that got run over by a truck (after the truck), but I don't think that mattered a lot because I am tone deaf anyway. I can tell you, it was not an easy instrument; whether anything else would have been easier, I don't know, but at least we didn't have to put a lot of money into it to find out that band in general and the french horn in particular were not for me.
The first parts of learning will be about finger positions and blowing and coordinating the two together and it won't matter if it's dented.
Good luck!
