technically there really is a difference. Shoal is a collective noun for fish. The term "shoal" applies for any group of fish, including mixed-species groups. "School" is for more closely knit groups of the same species swimming in a highly synchronized and polarized manner, essentially taking on the appearance of being one in motion and purpose. Both schooling and shoaling are socially important to the psychology of the fish and both are used to increase longevity in a world full of predators.
