I've seen both your posts and Im a little worried for your safety here. I'm in the UK and I know some things work a bit different in the USA so please bear that in mind.

Im not gay but I know what it's like to be bullied at school. I also know how easily insults can lead to violence, especially when there's a gang of bullies egging each other on. From what you've said I think this is more likely as this guy is obviously gay or bi and can't deal with it.

If I were you I'd be documenting all this abuse, saving messages and videos were possible. You need to make the school aware of what's going on. The best way would be to go with a parent and see the Principle. If nothing else they can keep an eye out if any trouble from this gang of lads. If the worst does happen they'll then have no excuse but to come down on them like a ton of bricks.

I'd also let his parents know about all this Instagram stuff. If they in any way doubted you then there is a video confession from him.

You've said you've gone looking for legal advice about the necklace. As it was a gift that's probably going to make it hard to get back legally. I think your only hope there is if he caves into pressure from his parents. However what you need to do is cover yourself from any action from him and his gang of mates. I'd find out if any of this is covered by hate speech or harassment laws where you are, I'd also look into restraining orders against them especially if you can find any evidence that any of them have posted something about violence against gays.

Last thing don't dispair. School can be rough. Things get better when you leave. Whether you go to Uni or straight into work it's life is completely different. You can find your tribe and things will get much better for you.