I started when one of my teachers allowed us to take said A5 sheet of paper into my A-Level mock exams. All the class did well, as we had this lifeline. Next exam we were given the same instruction but this time, unbeknownst to us until we walked in, our paper was taken away from us at the door. We were absolutely furious; until the teacher later showed us that not only were our marks all but identical to that of the exam where we had the paper, they were significantly higher than the exams we had before the whole cheat sheet debacle. Basically he tricked us into revising effectively. It's a technique that's stayed with me to this day.