The door to Mr. Sinusas' room is always locked. This allows him to quickly close the door and have it automatically be locked, to help keep everyone safe in the event of a fire or lockdown. If you are late to class or need to leave, you might be stuck in the hallway if the door is closed. A quick knock will draw attention and Mr. Sinusas can open it for you. If you are close to the door and hear someone knock, you can certainly let them in.