Get to the school. When you get to the room with the piano, first press the 3rd key (White), then press the 10th key (White). Then press the 11th key (Black), then press the 8th key (White). Now press the 2nd key (Black). They should all have been dead keys. The Silver Medallion should now have been released.
Important note: you should already have the Gold Medallion inserted into the clock tower for the Medallion to be accessible.