Class pictures are two days later. The fifth grade class gets their picture taken last, which gives Nick and his friends time to whisper something to all of their classmates. After the photographer arranges the class and tells them to say cheese, every kid says "frindle!" and holds up a pen. It's the only photo the photographer can take of the class, as he's out of film for his camera. Mrs. Granger is furious.
"Ruining" the class photo like this is another way that Nick demonstrates his understanding of the power of working together. It's impossible for the teachers to single out one kid, which means that Nick's entire class is safe from punishment and more powerful than their teachers.
This gets everyone at Lincoln Elementary using Nick's new word, which they all like a lot. The day after the school photo, Mrs. Granger posts a notice on the bulletin board saying that any student who uses "frindle" instead of "pen" will stay after school and write lines. This makes all the students just want to use the word more, and serving detention with Mrs. Granger becomes a badge of honor. She has kids serving detention with her for weeks.
Finally, at the end of seventh period one day, Mrs. Granger asks Nick to talk for a moment. Nick feels like a general participating in a war conference. Mrs. Granger asks Nick if he thinks the "frindle" thing has gone far enough, but Nick insists that "frindle" isn't a bad word and he's just putting Mrs. Granger's lessons about how words change into practice. Mrs. Granger sighs and admits that Nick is correct, but she says that "pen" already has a rich history and makes sense.
Nick understands that he's doing exactly what he's supposed to be doing as a student by applying what he's learning in the classroom to the real world. The fact that Mrs. Granger does admit that this is true suggests that she may be more sympathetic to Nick's cause than she lets on.
Nick innocently says that "frindle" makes sense too and points out that someone probably made up "pinna," that Latin root word for "pen," out of the blue as well. He explains that he and some friends took an oath to only use "frindle," and says again that there's nothing wrong with the word. Mrs. Granger seems unsurprised by Nick's answer. She pulls an envelope out of her desk and explains that it contains a letter she wrote to Nick, and he won't get it until this whole thing is over. She asks him to sign and date the back, so he'll know that she hasn't changed anything when he does receive the letter. Nick realizes that Mrs. Granger is enjoying the war and wants to win.
Keep in mind that while Nick doesn't know it, Mrs. Granger is choosing to fight Nick over "frindle." She does this because she recognizes that Nick enjoys pushing back on systems of power. If she were to give in, there would be nothing for Nick to push against and he might lose interest. Therefore, it's necessary for her to take this stand and make Nick feel as though there's an actual fight going on.
The next day, one of Nick's friends suggests that they get every fifth grader to ask Mrs. Granger directly for a frindle. Nick reasons that it's a great idea, since she can't possibly keep every student after school. Mrs. Granger keeps 80 students that day, and Mrs. Chatham has to stay to help manage all the students. The fifth graders do it again the next day, along with some younger students. Over 200 kids stay after school. Parents complain, the school board and superintendent get involved, and Mrs. Chatham visits Nick, Mr. Allen, and Mrs. Allen.
When the complaints of parents and the administration results in Mrs. Chatham deciding to do something about "frindle," it again shows that there's power in working together as a group to force change. Notice that Mrs. Granger has no qualms about keeping so many students and punishing them; by doing this, she allows them to also feel a sense of camaraderie.