In the office

You follow her very carefully, so she does not notice you. She stops at the Headmaster's Office and knocks, but nobody answers (of course). The room isn't locked, so she goes inside, leaving the door open.

You know very well that the Director will not come, even if she had made an apointment with her. So you pick up a rope and a rag & sneak carefully into the office, peeking through the door.

Your former lover is sitting in an office chair, looking at some papers, her back towards you. Perfect.

You manage to take her by surprise- she does not even see you. You throw the rag over her mouth and tie it behind her head, cleave-gagging her. Before she really knows what's happening, you have tied her from behind to the chair with the rope.

Maybe she does not even recognize who just did this to her - which promises much more fun. You close the door behind you.