Warlord's Revenge by Zac Harrison

When John Riley's class is chosen to be inspected by the mysterious scholars of Kerallin, they take an amazing trip to the scholars' hidden planet! As if being quizzed by the scholars isn't stressful enough, the planet is invaded by an intergalactic warlord! Expelled from Hyperspace High long ago, the warlord is out for revenge. Should the class flee while they still can, or can John and his friends save the day?