One wall of the classroom has knee-to-ceiling windows. She has visualized breaking one of those windows with a chair, and has a heavy rug from the classroom that she could throw over the opening to keep anyone from getting cut as they climb out, if they need to flee for any reason.
Windows that big might be tempered glass. Have her get a spring-loaded center punch to break it. They are about $5 and about the size of pencil. (useful for starting wood screws too)
What kind of door knob? Maybe some way slip a 2x6 with a slot in it over/through the door handle and wedge it against the door frame? (would be a lot easier if the door had steel hooks attached to to accept a 2x4) This could be a lot faster and stronger than using para-cord. OTOH, it could be used by a crazy student to barricade himself in the room.