What is the term for a group of fish swimming together?

The term for a group of fish swimming together is a school.

In everyday English, a school is a coordinated group whose members swim in roughly the same direction and maintain organized spacing. The behavior can look almost choreographed: fish adjust their speed and position rapidly in response to their neighbors, allowing the group to turn, expand, or contract without constant collisions.

Biologists distinguish a school from a shoal. A shoal is any social group of fish that stays together, even when the individuals swim or forage more independently. When the group becomes tightly coordinated and moves in the same direction, it is described as schooling. In ordinary conversation, however, “school” is the familiar collective term and the fair answer to this question.

Swimming in groups can improve predator detection, confuse attackers, and reduce the chance that any one fish will be captured. It may also help fish find food and, in some conditions, reduce the energy cost of swimming. Species such as herring, anchovies, sardines, and tuna are famous for forming large schools. A school is not the same as a flock, pack, or herd, terms generally associated with birds, mammals, or hoofed animals.