The Scholomance Tome 1
A Deadly Education

Enter a school of magic unlike any you have ever encountered. There are no teachers, no holidays, friendships are purely strategic, and the odds of survival are never equal. Once you're inside, there are only two ways out : you graduate or you die. El Higgins is uniquely prepared for the school's many dangers. She may be without allies, but she possesses a dark power strong enough to level mountains and wipe out untold millions - never mind easily destroy the countless monsters that prowl the school.
Except, she might accidentally kill all the other students too. So El is trying her hardest not to use it ... that is, unless she has no other choice.