
Go to the Second Floor and stand behind the large statue of Gregory The Smarmy. As Harry, look up and target the candlestick. You may need to stand back to do this -- be careful if it is at nighttime, as the Prefect may see you. Use Carpe Retractum on the candle, and on your left a secret room with a chest will open.
Shortcut to the fourth floor in Gryffindor Tower
Go to Gryffindor tower and play as Ron.