The School for Humans: (The Book of Lost Caleans, part 1) by Vanessa Kittle

Our epic journey begins in Bronze Age Ireland.
Ellene is a servant to one of the great Calean leaders. The Caleans are opening a school for the young from Ellene’s village, but not all of the Caleans want to teach the humans. Some have much darker plans, and the school may be the last chance for peace between the two peoples.