The Keepers of the Vault Series by Marty Chan

On the fourth floor of her haunted school, Kristina discovers a vault of rare artefacts gathered throughout history. In the wrong hands, these treasures can unleash evil on the world, and dark forces will stop at nothing to acquire them. Kristina and her friends must risk life and limb to keep the vault and its contents safe.

Books in the Keeper of the Vault Series:

Keepers of the Vault: Fire and Glass
Keepers of the Vault: Melody and Myth
Keepers of the Vault: Shadow and Spell