The secret room can be found on the final level of the game. After breaking the first door after the elevator, drop to the right where the bathroom door is. Inside the bathroom break the two windows to get into a sideways version of the first level. Across the gap in front of you there is a ledge, just run forward to land on it. After that, you will see two desks on the other side of the hallway. Run forward and try to land on the lower desk. Looking at the wall on the other side you will see a "sideways" door. Run towards it and try to land on the ledge of the doorway and break the door to get in.