A group of fish can be called a school or a shoal. Both words from the Dutch root "_schole_" meaning a troop or a crowd.

From [Etymology Online][1]:

> "group of fish," late 14c., *scole*, from Middle Dutch *schole* (Dutch school) "group of fish or other animals," cognate with Old English *scolu* "band, troop, crowd of fish," from West Germanic *_skulo_- (source also of Old Saxon scola "troop, multitude," West Frisian *skoal*), perhaps with a literal sense of "division," from PIE root *skel- (1) "to cut."


  [1]: https://www.etymonline.com/word/school#etymonline_v_22908