Class Pest by Davy Ocean

In the second book of a Shark School series spin-off, young hammerhead shark Harvey Hammer's adventures continue! Harvey can't believe his good—and bad—luck! The good? He's invited to Flash's awesome birthday party! The bad? At the same time as the party, he has to take care of Pirate the Parrotfish, a pesky class pet, who loves nothing more than getting Harvey into lots of deep-sea trouble.