So this idea came to me while listening to the news although the actual event was quite disturbing I think it would make a good rp so then to the idea.
Two girls, sisters (Thinking around 16 - 19) are sent to a dancing. school. Supposedly a very prestigious school, the two of them hit it off right away with the other students and the teacher. As things go on they start to notice things like how he touches the other students in ways that don't seem right. After a while he starts touching them as well. And things begin to escalate from there. The girls are forced to wear more tight and revealing outfits. Each day a different girl is chosen to stay back after class where they are submitted to his desires (although only the girls that have previously been chosen know what happens) eventually one of the sisters is chosen to stay back and he assures the mother it's just for some private lessons.
He undresses her, gets her to do different dances for him, gets her to pose for pictures all under the guise of extra lessons. Eventually he would fuck them as well as many other twisted desires he wishes to enact upon his students. A possible end point for this would be the girls sold as prostitutes. But that's up to you. Ages are negotiable.
Along the same vein I'd like to do a similar story with a gym class, perhaps a cheerleader or a sports star? The gym teacher is very hands on eventually leading to full on sexual activities with her.