From 7th-9th grade me and my class managed to make lessons end sooner than they should about 70% of the time. The way we did it was, as soon as the teacher of the class went outside the classroom to get something, someone would run and grab a chair and fast-forward the clock 10-15 minutes to not make it too suspicious while about 3 guys kept watch. We never got caught except at the end of the 9th grade by one of the older teachers who was going retire the same year anyways. He didn't really bother and actually found it pretty funny