If you have a big predator in your tank that terrorizes (and is ready to eat) everyone else, you will see all other fishes school.
Schooling behavior is a natural defensive instinct for reef fish. Fishes form school to deter large predatory fishes. For damsels, they will school for the first couple of weeks to months, but after they realized they are in no danger in the tank, schooling no longer visible, or at most, loosely formed.