As of version 0.36, Treasure Chests were added to Adventure High. They can be found in the Dungeon.
There's currently two, one of 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 twelve, 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.
For dialogue related to the Treasure Chests, refer to the Treasure Chest Transcript.
The second treasure chest requires Willing Slave Cassandra in the party.
This treasure is on floor thirteen, in the top left corner.