If it is the second prefect that is standing on the corner of a square hallway room, then I know how to help you! If the prefect is in the bottom left corner of the room when you enter in the top left, walk around the hallway to the right and on one of the walls (before you pass the desk) there is a painting on the wall. There wont be a blue orb around him to talk to him, but if you tap on him, he says that he will distract the prefect for you to get by!
Also, ps, if you turn the corner and start walking on the bottom of the square room, from right to left, towards the prefect, you'll be able to find a hidden chest!