Northern Lights (His Dark Materials, #1)

by Philip Pullman

In fact, of course, Lyra and her peers were engaged in deadly warfare. First, the children (young servants, and the children of servants, and Lyra) of one college waged war on those of another. But this enmity was swept aside when the town children attacked a colleger: then all the colleges banded together and went into battle against the townies. This rivalry was hundreds of years old, and very deep and satisfying.