Hey @Worriedmum123, never be worried about posting in the wrong area, we can always move your post to a different area if need be 
Wow, it sounds like a really difficult situation for both you and your son. How is your son coping with it all? I'm wondering how he sees his part in it? I hope you don't mind me asking that, and I am by no means blaming him at all! I think it's such a shame the other parents blame your son and are not looking at the behaviour of their boy. It's could be such a great learning experience for them both.
My daughter went through a similar situation in year 6. Two girls who had been her friends posted a youtube video making fun of her mental illness. I rang both parents. One brought her daughter over to apologise and brought a gift for my daughter and they hung out for a little while. The other became very angry with me and there was never any apology or acknowledgement from them for what their child had posted online. The daughter herself apologised about a year later.
It's merely my opinion, but I think making and posting a threatening video online sounds more serious than a comment, and I'm wondering if the school was shown the video? I can understand your upset and frustration, especially when it sounds like you are acknowledging your son's part. I think it's pefectly normal to feel the way you do. That frustration and feeling of injustice can be hard to let go of can't it. We want this world to be a fair one for our kids.
I haven't found the answer for letting go of things that grate on me, however, I find that if I focus on what I can control, I have less time to be annoyed about the things I can't control - which is other people and how they treat me or my daughter. You are doing the right thing by monitoring your son, and that's all you can do. You can also talk to him about his part in it, so hopefully he can learn too, and it sounds you have already done that. You're obvioulsy a very loving mum.
You could also follow up with the principal and discuss with them their reasons for blaming your son. There may be further action you can take from that angle if you're not satisfied with the response or lack of action.
