What was that book about a class without a teacher?

Okay, so I'm in high school right now, and I read this book back in elementary school, but I can't remember the title. It's about this class spending the day without a teacher. At first it's fun, but of course, things get bad. I remember there was this one girl in the class who had been refusing to talk, but suddenly spoke in the book. I think the book was told from the POV of a young boy. On the cover was this chalkboard with the title written on it, but I'm blanking on what that was. If anyone knows what I'm talking about, help would be much appreciated.