Failing a class or a year is highly disruptive all around. If a large number of students literally lose the ability to write original work come test day, is the school going to fail half the student body? No, that’s crazy talk. It would be considered a disaster by the school, the parents, and the wider society if it was widespread. Schools put rules in place to ensure that your work is your own mainly to prevent children from doing stupid things that will make them fail on test day.