Mutant rat attack!

"Third-grader Dexter Drabner dreams of being a skateboarding-hero-spy, while contending with his nemesis at school, Millicent the bully--and when science teacher Mr. McFur's prized lab rat, Pretty, eats a piece of radioactive gamma broccoli and grows to the size of an elephant, he gets his chance to save the school from destruction"--OCLC.