First off, is she going to shoot trap or skeet?
The two are words apart in regards to shotguns.
For real trap, if she is not going to shot doubles, a BT-99 would be the right choice (less than a $1000 used). If doubles, then you better start saving you lunch money, with combo O/U starts around $2000 used and goes up from there (read 682 being around the cheapest O/U combo you will find).
As for real skeet, depending on the school team (read if they just shoot one gauge or all four) this will dictate the shotgun. If she needs to be rig'd up in 12, 20, 28, and .410, then your looking at an O/U with a tube set. A Briley tube set will set you back around a C note, so with a descent used shotgun like again, the 682, your over the $2500 mark buying a used O/U and a tube set.
If they are just shooting what shotguns they have (read 12 or 20 for both trap and skeet), then this could be solved by buying her a youth model berretta 391 in 20 gauge (read perfect shotgun for her fame and size). The reason that I bring up this shotgun is that the stock is adjustable, and since the barrel uses screw in chokes, this shotgun could be used both for trap and skeet for her (not formal, but would be a good shotgun to get her started until she settles into one of the other). The one down fall that does come to mind in regards to using a 20 for trap is depending on how they are shooting classes (read either as a personal standing or a team standing like in league), she could find here self back at the 27 yard handicap position, and to go clean for a round using a 20 gauge, she will have her work cut out.
To sum it up, you really need to spend some time taking with the school coach, and together, I'm sure that you will be find a shotgun best suited for her to both start out, and when the times comes, put a huge dent in the check book buying her that Perazzi, Kolar, or Krieghoff that she will just have to have someday.
(Note, Mike is going to die laughing when he read this due to the fact that HKgnnr is clueless to the amount of money he can drop into the sport really fast. Hell, she may be his little daring, but consider that I shoot will college and high school shooters all the time,and these guys are rig'd up with the right rigs for the events, it's going to take more than his front pocket money to keep her up and running for the season.)