Nina and the Keeper of the Keystone

In book two, Nina and her friends realize they can use the magical mailbox to help their fellow students deal with their problems. Soon there's a stream of kids looking for advice.

Not wanting to divulge their secret to other students, they say that they merely draw on history to give their advice. But when a student takes the advice from a letter Marie Curie sent a little too literally, almost blowing up the science wing, their operation is forced underground. What's worse, a new principal arrives on a mission to improve standards, and he seems to know a little more about Ada Lovelace Middle School than he's letting on . . .