You have to defeat General Akilles in Cyclops and speak to Nolan Stormgate who will want you to deliver the message to the headmaster. Defeat Alicane Swiftarrow and talk to Private Quinn who wants you to relay the message as well. Then beat the Harvest Lord and talk to Susie Gryphonbane who sends you on your way to the headmaster. It doesn't matter the order in which you defeat the bosses.
When you speak to Ambrose he wants you to fight Foulgaze in Sappyra's tower to get the key to nightside and be on your way to fight Lord Nightshade in the storm tower.
Ambrose will also tell you to speak to the guard (Doyle?) at Colossus Boulevard at this time.