High school dances always have at least one chaperone to keep an eye on things. A chaperone is someone who looks after and supervises another person or a group of people.
Chaperone can also be spelled chaperon, without the e. It originally meant a woman whose duty was to accompany a younger woman and make sure she wasn't harmed and didn't get into trouble, especially when she was with a man. These days, chaperone is most often used to describe an adult who supervises a group of kids or teenagers. The Old English meaning was "protective hood or head covering," and today the word chaperone retains that "protective" quality.