We the Children, by Andrew Clements

Benjamin Pratt's life has been difficult lately, with his parents' separation, but it takes an unexpected turn when the dying school janitor presses a gold coin into his hand and extacts Benjamin's promise to defend and protect the school. The Duncan Oakes School has been sold to a developer for an amusement park, but now it is up to Benjamin and his friend, Jill, to try and save the school and preserve their town as they know it. What is the secret that Duncan Oakes hid in the school and can they find it in time? This is Book 1 of the "Benjamin Pratt and the Keepers of the School" series. 2010
R. Rauch