The Red School is group of several small red fish with a single light stripe each.
Red Schools only appear in the underwater sections of Aquarium Park, usually around Air Bubbles. When Sonic touches a Red School, they will surround him. While around Sonic, the Red School will prevent Sonic from drowning and once negate the damage Sonic would receive from an enemy attack, like the Aqua Shield. However, negating damage will cause the Red School to disband.
The Red School will only remain around Sonic for twenty seconds or until Sonic resurfaces, though this period can be extended if the player touches another Red School.