What is a group of fish called?

A school, and biologists distinguish it from a shoal. A shoal is simply fish hanging around together for company; a school is a shoal that has organised, all facing the same way, matching speed and spacing with the precision of a formation team. Roughly a quarter of fish species school all their lives.

There is no leader and nothing coordinating it. Each fish follows a handful of simple rules about its immediate neighbours - keep this far away, match their heading, move towards them if they get too distant - and the collective behaviour emerges from that. Turn one fish and the change propagates through the school faster than any individual could have reacted to a signal.

The point is confusion. A predator has to pick one target out of a moving mass of identical bodies, and the shifting shape makes it hard to fix on any of them; the odds of being the one taken also drop simply by being one of many. The same three rules, written into software, are what produce convincing flocks in films - the algorithm was published in 1986 and its author called them boids.