This is called Findel

Nick, one of the most daring children in the fifth grade, is a master at disrupting the class with inappropriate questions. However, his tough grammar teacher does not play into his hands and asks him to prepare a report on the origin of words. Nick, impressed by what he has learned, comes up with a brilliant idea to test it out: he starts saying frindle instead of pen. The word, which has no meaning, spreads quickly in the school and then in the town, becoming the talk of the town. Nick's innocent game will lead to unexpected results.