The Golden Enclaves by Naomi Novik TPB

Saving the world is a test no school of magic can prepare you for in the triumphant conclusion to the Sunday Times bestselling trilogy that began with A Deadly Education and The Last Graduate. The one thing you never talk about while you're in the Scholomance is what you'll do when you get out - not even the richest enclaver would tempt fate that way. But that impossible dream has somehow come true for El and her classmates. And what's more, she didn't even have to become the monstrous dark witch she's prophesised to become to make it happen. Instead of killing enclavers, she saved them, and now the world is safe for all wizards. Peace and harmony have enveloped all the enclaves of the world. Just kidding. Instead, someone else has picked up the project of destroying enclaves in El's stead, and everyone she saved is at risk again with a full-scale enclave war on the horizon. And so, the first thing El needs to do after miraculously escaping the Scholomance, is to turn straight around and find a way back in.