The invasion of plastic balls
Plastic happens to be the natural enemy of magic.
In "Saving Wiztech (Part 1)", Wiztech is condemed after the mysterious infestation of plastic balls throughout the whole school, as it enabled any usage of magic. While Justin was occuypying the school in order to find to what caused the infestation, he finds out that Ronald Longcaped Sr. was the one capable of manifesting Wiztech with plastic balls (in which his plan was just throwing the ultimate birthday bash for himself). Justin attempted to use his magic to capture him but because of the plastic balls Ronald Longcaped Sr. got away. Justin later gets the executives of Wiz World Industries to remove the plastic balls, in exchange for their products being endorsed at the school.
Wizards cannot get their magic through plastic, either. In "Night at the Lazerama", plastic is once again considered a hazard when Justin and Juliet get trapped in the mummy exhibit, and Justin is unable to use magic to get them out because of the plastic wall display.
The exact reason why plastic doesn't cooperate with magic is unknown.