As of version 0.36, Treasure Chests were added to Adventure High. They can be found in the Dungeon.
There's currently one, which requires you to have Doll Sarah in your party.
In other words, you must have turned Sarah into a doll, then used Boost to change her back to normal.
From the entrance to floor 12, head right, down, left and straight up. It's in the right most "room".
It contains Sarah's clothes, though they're more pink and quite revealing.