the.com/school of fish

means A large group of fish swimming together in a coordinated, often synchronized way, typically for safety, feeding, or migration.

from The fish sense has nothing to do with schoolhouses or learning — it's a separate borrowing from Middle Dutch 'schole,' meaning a troop or multitude, related to the Dutch word that also gave English 'shoal.' Both 'school' (the gathering of fish) and 'shoal' (a crowd, or shallow water) trace back to the same Germanic root for a large assembled group. The education sense of 'school' comes from an entirely different source, Latin 'schola,' and Greek 'skhole' meaning leisure or discussion — so the fish and the classroom are etymological strangers who just happen to share a spelling.