As with any piece of literature, each piece—whether a novel, play or short story—offers varying themes to the reader (or audience), and different people come away with different interpretations. In Andrew Clements' novel, Frindle, there are several themes or "lessons learned."
One lesson reflects the old saying, "Be careful" what you wish for." In this case, Nick is always looking for ways to disrupt class. However, when he introduces the word "frindle" to mean "pen," the action takes on a life of its own. It spreads throughout the community and out into the world, ending up on a line of products, on David Letterman's talk show, and in People magazine. While Nick gets what he wants, he is increasingly uncomfortable with the enormous proportions this "small," seemingly meaningless word takes on, and how its power becomes even more than he can control.
Another theme or "lesson learned" is that things aren't always as they appear. This is a common theme in literature—we do not fully understand the extent of it until much later in the book in the form of a letter Mrs. Granger had shown Nick much earlier, which she finally sends to Nick. After he has graduated high school and college, he receives the "mysterious" letter Mrs. Granger had written—to which she had asked Nick to affix his signature at the seal, to show that the letter had not been opened. The purpose of this step is to show Nick what her intentions were when the entire "frindle" movement began. She wanted him to know what while she had at first been angry by his "attack" of the English language (something dear to her heart), she soon came to support him. (This is foreshadowed when Nick starts to become upset by the attention "frindle" receives and Mrs. Granger's goes out of her way to calm him, showing her genuine concern for him.)
A third lesson learned is that sometimes something that may be done for the wrong reasons (like driving a teacher crazy) can become something good and worthwhile. When an idea leaves us, as is the case with artists and philosophers, it takes on a life of its own. Sometimes something valuable comes of it: Nick ultimately decides to take steps "to reform the school lunch program." Second, he money from the "frindle" merchandising provides him with a trust fund that he uses to help his parents and then to "endow a scholarship" in Mrs. Granger's name.
Finally, we see exactly what Mrs. Granger was trying to get Nick to see: that language is a powerful tool. Used for good or "evil," language has a power all its own: the power to change the world. (Note, for example, the power of the Declaration of Independence, or Abraham Lincoln's Emancipation Proclamation.) It is amazing but true as the author demonstrates the power a small word like "frindle" takes on—changing the lives of many people who come in contact with it, especially the "uncooperative" Nick that Mrs. Granger first meets.