Much to Mrs. Granger's evident annoyance, the word frindle—meaning pen—has taken off in a big way. Pretty much all of the kids at Lincoln Elementary have started using the word, a word completely made up by the book's protagonist, Nick Allen.
Even when it comes to taking the group photo of everyone in the fifth grade, the word isn't far away. Nick and his friends have been hard at work, whispering into the ears of every fifth grader exactly what they want them to do when they come to have their group photo taken.
All the fifth graders are lined up on the auditorium stage, ready for their group photo. But when the photographer tells them to say "cheese," they say "frindle" instead and hold up pens—which, of course, are known as frindles—to the camera.
As the photographer is out of film, this is the only photo of the fifth-grade he's able to take. As one can imagine, Mrs. Granger, Nick's antagonist, is absolutely furious about all this and vows to punish anyone heard using the word frindle instead of pen.
In fairness to the kids, they didn't deliberately set out to annoy the teachers; it was just a bit of fun. But whatever their intentions, they've got people talking about the new word, which suits Nick just fine.