Mistake of Magic by Alex Lidell

Three trials stand between Lera and exile. Unless the training kills her first.
Quint magic has never chosen a human before, and the Elders Council is convinced Lera is a mistake.
When the quint refuses to be cleaved apart, the enraged elders give them a choice: exile from Lunos or demotion to the lowest of trainees. Subject again to the humiliations and deadly trials they endured centuries ago, the males now face a new challenge–training Lera to survive.
River, Shade, Coal, and Tye will do whatever it takes to keep Lera safe. But Lera will do whatever she must to keep them together–even if it means putting herself in mortal danger.