Night Patrol

The Turning Stair6×66 prefects7 rooms

A basilisk came out of one of the rooms last night. The prefects were standing at their posts and saw nothing, which means the snake never crossed a room they were watching. There are seven rooms and only six prefects, so one room was left without a post. Put every prefect back where they stood and the empty room gives itself away.

Rules

• Place all six prefects. Exactly one arrangement fits, and the puzzle solves itself the moment you find it.
• Every line in Filch's notes is true. The work is in what they rule out.
• No two prefects stand in the same row.
• No two prefects stand in the same column.
• No room holds more than one prefect.