What is the Difference between a school and shoal of fish?

The difference between school and shoal depends on what the fish are doing and the reason for them being together. A group of fish (usually but not necessarily of the same species) which are simply together, but not acting in a coordinated way, is a shoal: for example they may be moving in different directions, some eating, some not, and so on. A group of fish acting in a coordinated way (almost invariably of the same species) is a school: for example a lot of fish of one species migrating to a different area, or moving together on an erratic path or forming a "ball" to avoid a predator.

cf: a crowd of seagulls at the seaside, all doing diffferent things, would be a shoal; whereas a flock of knots wheeling in the sky would be a school.

(Mnemonic: school = discipline !)