The Great Influenza: The Story of the Deadliest Pandemic in History by John M. Barry

Saving the world is a test no school of magic can prepare you for in the triumphant conclusion to the New York 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 it's all we dream about, the hideously slim chance
we'll survive to make it out the gates and improbably find ourselves
with a life ahead of us, a life outside the Scholomance halls.
And now the impossible dream has come true. I'm out, we're all
out--and I didn't even have to turn into a monstrous dark witch to make
it happen. So much for my great-grandmother's prophecy of doom and
destruction. I didn't kill enclavers, I saved them. Me, and
Orion, and our allies. Our graduation plan worked to perfection: we
saved everyone and made the world safe for all wizards and brought peace
and harmony to all the enclaves of the world.
Ha, only joking!
Actually it's gone all wrong. Someone else has picked up the project of
destroying enclaves in my stead, and probably everyone we saved is
about to get killed in the brewing enclave war on the horizon. And the
first thing I've got to do now, having miraculously got out of the
Scholomance, is turn straight around and find a way back in.