Actually a shoal is a group of fish loosely hanging about together in no particular pattern, a school is a group of fish all facing and traveling in the same direction.
"shoal" means :1.great number of fish swimming together
2.shallow place in the sea;sandbank,esp one that can be seen
when the water lever is low.
3.hidden dangers or difficulties
The important thing is that they are both a group of fish, and that fish that are supposed to live as a shoal or a school should always be kept in higher numbers ;-)