Why is a group of fish called a 'school'?
Some fish tend to swim in groups in order to protect themselves from their predators. The collective noun which denotes such a group depends on the species of the fish in the group, as exemplified by the terms a troop of dogfish, a swarm of eels and a glide of flying fish.
The most common collective nouns for a group of fish in general are school and shoal.
See Also
Weird and wonderful collective nouns12 Facts About Otters for Sea Otter Awareness Week | U.S. Department of the InteriorWhat is a group of sloths called? | The Sloth Conservation FoundationA gaggle, a confusion and a conspiracy - bizarre animal collective group namesBoth the words have evolved from the same common Dutch root 'schole' meaning a troop or crowd.
— V Venkata Rao, Ahmedabad
See Also
Cephalopods