The "detention" idea is a fantastic one and started off my campaign wonderfully. I added a twist: when the characters entered detention there was a substitute teacher there - a disgustingly fat, bald man in an all-white suit and vest that had food stains all over it. He smelled awful and was very stern with them, calling himself Mr Eggder. He left the room while threatening them if they moved from their seats.
Cue the water rising - I added fish, sharks, whales and something even larger swimming in the distance. The windows begain to crack, prompting their quickly leaving as the windows shattered and water rushed in. A great scene ensued as the kids made their way to higher ground (the 3rd floor), ocean-like flooding threatening to drown them. They eventually had to enter the forbidden Teacher's Lounge to escape, which opened up into the Great and Awful Forest.
Jock: a bottomless backpack
Dreamer: a diary that wrote hints when they were stuck
Bully, a baseball cap that became a distance weapon and boomeranged back
Their first wacky adventure: Just after entering the Great and Awful Forest they encountered Luck Bear of the Care Bears being chased by a dozen rabid, mangy rabbits that only had one foot each (and 2 paws), hopping crazily as if on mini-pogo-sticks. The rabbits, who had all lost a foot to people wanting lucky-foot keychains, wanted to steal Luck Bear's luck from him.
The intent was that the kids would save Luck Bear and he would become a guide and mentor for them at the begining of their adventures, lending his considerable luck to their cause. Especially since the rabbits were minions of the Rotten King and some got away to inform him that there were new children in GRIMM.
However, after saving Luck Bear the Jock stuffed him in his bottomless bag with only his head sticking out and proceeded to interrogate him threateningly. After pointedly not taking a series of hints that they should treat him more respectfully, Luck Bear used his luck to escape (thereby ruining the bottomless bag) and left them high and dry and on their own.
My overall campaign revolves around them having to find 8 magical keys that will lead them to Babylon. Each key they find will graduate them to the next Grade. What and where the keys are, and what exactly Babylon is, has yet to be decided.