They should have killed the teacher by stomping on him while he was down. He might be strong, but if he's an average person then he can't handle 4 teenagers at the same time when he's not in a defensive position (even if he knew Judo, which has defensive positions on the ground: he was knocked down anyway, so it doesn't matter). Then they could use that recording as an alibi that they prevented a rape, but didn't know when to stop because they were dealing with an adult.
Worst case scenario, they get sent to juvie (I guess they won't because they were preventing their own rape, and didn't know when to stop because they were dealing with an adult). They should call 911 for good measure, but make sure he can't survive. Possibly the charges would get dropped because they are children fighting against their abuser, and they have a lot of evidence against him (with the recording) with dozens of eye witnesses outside. Maybe they could get counseling, that'd be the best case scenario.
Where is the conclusion? Did that perverted teacher get arrested for assault and child abuse?
They should have used that gap to kill him.