What is a school of whales?

The Word school (or shoal) isn't defined by a specific number but simply means 'a large group or multitude', similar to Words such as flock (of sheep/birds), gaggle (of geese), herd(of cattle), swarm (of bees), pack (of hounds), business (of ferrets), mob (of kangaroos), mess (of iguanas), pride(of lions), plague (of locusts) or murder (of crows). For a comprehensive list of collective names for groups of animals go to AskOxford.com