A schoal of fish...
"Flock" is usually birds
Shoal: A simple group of fish is called a "shoal."It can contain a number of species. ... School: A group of the same species of fish that swim together and move in a synchronized manner is referred to as a "school."
Shoal of fish ?
yes, shoal of fish is correct. "School" refers to whales or dolphins (sometimes sharks)
Why not a flock of fish?
Because English has arbitrary words to refer to groups of specific animals.
a shoal of fish, a school of dolphins