What? Just regular fish.
Did you mean 'why'?
Why Is a Group of Fish Called School?
Um, there are two "school"s in English. Same writing, same pronunciation, but a bit different meaning and origin.
The first "school" that we use to refer an education institution is a Greek origin.
Skholē, meaning idle time. Well, they discussed something in free time. It was their favorite as there was no computer monitor to glare at, yet. The word then becomes "school" in English.
The second "school" is maybe a Proto-Indo-European origin, prefix "skel-". Meaning "to cut", broadened as a noun to refer a "division". The word then becomes "school" in English. This second one means "group" (specifically for sea animals).
One more thing, the second "school", which refers to "group" (of marine animals), appears about couple hundred years earlier than the first "school", the place to educate young people.