This is A School for Sorcery.
A School for Sorcery is a 2002 fantasy novel by E. Rose Sabin. Tria, the daughter of a farmer, receives an invitation to the Simonton School for the Magically Gifted. Excited to be able to use her talent, she travels to the school only to find that the glorious buildings of the brochure are dilapidated, and that most of the instruction is not on how to use one's gift, but on philosophy and theory of why it should not be used. Complicating things, her roommate is a spoiled rich girl who has no qualms about using her power with reckless abandon with no hint of the staff trying to quell her and there's a sinister conspiracy among the students to take over the school. It was followed by A Perilous Power in 2004, a prequel about the establishment of the school, and When the Beast Ravens in 2005, which follows one of the victims of the first book as he recovers.
One of the ways to achieve more power (initially used by the villain) is to create a copy of yourself in a mirror, then steal their power.
Tria winds up making three copies of herself, and it's one of the copies that survives to the end of the book. It also has her accidentally create a world in a tear drop, with her first task as a graduate is to keep that world alive, with the underlying message being that the moment she fails to maintain it, the entire world will cease to be.