Mirror Mischief by Ceci Jenkinson
Magic Trouble! Meany maths teacher Vernon Surd punishes anyone who can't do fractions (= Oli). School bully Slugger Stubbins punishes anyone who dodges his rugby tackles (= Oli). But when Oli uses his new magic mirror to punish his punishers, they go absolutely wild! Now only Skipjack can save his friend, with a bit of help from the Zombie Witch Doctor.