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Schools of Fish are groups of individual fish that swim together. In WoW, these are cylindrical regions in the water with individual fish swimming in circles. Some Schools of Fish are fishable fishing pools while other Schools of Fish are ambiance critters and are not fishable.
Schools of Fish are fishing pools that contain (primarily) fish. They are location objects found in bodies of water in WoW that have a higher portion of more desirable items that can be fished. Some fish can only be obtained from fishing pools. Schools of Fish can be found in both Azeroth and Outland, so presumably will also be found in new areas in future releases.
Fishing pool Schools of Fish are shallow cylindrical regions within fishable water, just under the surface, sharing a common top surface. They designate small regions within fishable water that give better kinds of catches than the surrounding water. The graphical representation is more than just the surface definition significant to fishing. You can swim though a School of Fish and see individual fishes swimming around at different depths within the pool.
See Fishing pools for fishing particulars and varieties.
Ambiance schools of fish
Schools of Fish can also be critters which are for ambiance only and have nothing to do with fishing. Two flavors exist, one that can be targeted that has a yellow (neutral) label, "School of Fish", and one that cannot be targeted and has no label. They can both be found swimming near each other in the same body of water; there does not appear to be a particular reason some are targetable and some are not. These swim deeper than the Fishing Schools of Fish and have no surface effect.
Even though these are made up of individual fish graphic models, they act as a single object. If you kill the targetable school of fish, each individual fish dies and rolls over on its back in unison.
(Non-targetable free swimming individual critter fish also exist.)
Fish critters of any sort seem to appear only in fishable waters. Absence does not mean the water is not fishable, but presence is a good indication that the water is fishable.