Practical Joke

At high school, some pupils decide to play a 'practical joke' on their teacher. On the day of midterm exam all of them say they she had told them the wrong date for the exam. She almost believed them, but then asked a girl known for her honesty and the girl could not lie. As a result everyone in the class gets an F for that midterm. That's a real problem for some of them - with an F they cannot have perfect GPA and will not enter prestige Universities. They want revenge...