Catholic school isn’t the greatest place to learn about the faith, so I wouldn’t expect a 15 y/o to know more than a practicing adult. Instead, look at her parents and whether they are actively a part of her faith formation outside of forcing her to attend Mass and school.
You could ask her to talk about how she feels and why, and then LISTEN. What is she frustrated by? What bothers her? What is she looking forward to most by leaving the faith? People at that age just want to be heard, and if she knows you are faithful, she may be more open to listen to you when/if she asks about anything.