Plot hole: In the opening scene, the bullies all go into the girls' toilets with loads of camera equipment, including high tech ones used for proper films and boom mics. This equipment is way too expensive (even more so in the 80s when the film is set) for any high school kid to have.
Plot hole: When the bullies take revenge on Marty, they doctor the chemicals he is preparing so that they explode. The bullies were punished some time in the afternoon, judging by the sun in the very first shot. Their detention is at 4:30 in the afternoon. Somehow, they manage to acquire the necessary chemicals for the explosion within a couple of hours, even though they are stuck at school. In addition to this, they have no way of knowing what chemicals Marty is preparing, so even if they were able to get chemicals, they would not know which ones to get to destroy his experiment.