The Scholar and the Last Faerie Door by H. G. Parry

All they needed to break the world was a door, and someone to open it.

Camford, 1920. Gilded and glittering, England’s secret magical academy is no place for Clover, a commoner with neither connections nor magical blood. She tells herself she has fought her way there only to find a cure for her brother Matthew, one of the few survivors of a faerie attack on the battlefields of WWI which left the doors to faerie country sealed, the study of its magic banned, and its victims cursed.
But when Clover catches the eye of golden boy Alden Lennox-Fontaine and his friends, doors that were previously closed to her are flung wide open, and she soon finds herself enmeshed in the seductive world of the country’s magical aristocrats. The summer she spends in Alden’s orbit leaves a fateful mark: months of joyous friendship and mutual study come crashing down when experiments go awry, and old secrets are unearthed.

Years later, when the faerie seals break, Clover knows it’s because of what they did. And she knows that she must seek the help of people she once called friends—and now doesn’t quite know what to call—if there’s any hope of saving the world as they know it.