Think about the early parts of the story and Mrs. Granger's war against Nick and his new word. Now that you know what her real intentions were—to intentionally make herself the bad guy—try to imagine what she was thinking when she posted her angry notice or when she kept all those students after school.

Mrs. Granger is a very strict fifth-grade language arts teacher who does not want any student to question her rules or cause trouble in the classroom. However, she figures that the other students will follow Nick’s rebellious nature, as it appears to give him the attention that he seeks. Mrs. Granger wants to keep order and control and be the authority figure for the students. But Nick wants to keep pursuing his goal of popularizing the word "frindle," and he repeatedly disrupts her classroom to do so.
Mrs. Granger insists that if the students want to use the word "frindle," they need to suffer the consequences and must stay after school. The students are choosing to follow Nick’s lead and do exactly that, and the kids stay after school. In addition, Mrs. Granger’s angry posting seems to have the opposite effect upon the students. Instead of getting her students to stop their behavior, it unites them in rebellion against Mrs. Granger’s request.
Mrs. Granger tells Nick later that, at first, she was upset with his behavior. Later, Mrs. Granger reveals in a letter to him that his quest to change the word "frindle" is rather interesting and that she wants to see that "frindle" becomes a commonly accepted word. She knows how words are created in the English language, and she sees that Nick’s word is taking shape, not unlike other words in history. In the end, you see that Mrs. Granger plays the villain to help the story of "frindle" become more popular than ever. Nick’s endeavors are successful, and he becomes rich and famous for inventing the word "frindle."
https://www.scholastic.com/teachers/lesson-plans/teaching-content/frindle-discussion-guide/