As a teacher of gifted kids, I would see Nick's behavior and the reaction of his classmates as a cue that he is special. I do not think that all of the adults in the school fully appreciate that, or try to really run with it. There are so many places this story could go!
I'd jump in as one of the interviewers on one of the shows Nick is a guest on. I'd tell him that all words have histories and deep meanings that span eras of human history. I'd encourage Nick to determine--if possible--the meanings of the parts of "frindle" and suggest he might want to modify the word somewhat based upon what he learns from searching the history of languages. I'd argue that there is an optimal meeting place for rebellion, creativity and history.
I have to agree that my landing place in the novel would be as Mrs. Granger. The place where Nick challenges Granger about the origination of words sounds like something I would do. The light-bulb moment is the most amazing thing in a teacher's life.
If I were to jump into the story of Frindle by Andrew Clements, I would probably want to get to be friends with Nick, but NOT because he becomes famous or rich. As an English teacher who loves language and the power of language, as Mrs. Granger does, I would find it gratifying to be there when Nick has grown up and has learned to appreciate the power of the little world he created: "frindle." In retrospect, I believe he would be amazed to learn that Mrs. Granger had been correct all along and that, ironically, he had proven what she had been trying to impart to her students, especially Nick, all along. It's not the "I told you so" that interests me, as much as watching the "light-bulb" go on in Nick's mind. For a teacher, it is an awesome moment.