You can record the IG live with your phone and take it to school. In some locations, what students do outside of school against other students is consider bullying. It doesn't have to happen inside the school for them to take actions.

I would go to a school counselor, give them the videos, tell them how they all did a dare and what this guy did (pretending to be your BF). They are all in school and this also went on in school too. Maybe pressure from all of that, he'll give you the necklace back. This can potentially go on their records and it would be shared with colleges.

That's what I would do.