No offence, but if your stern talk is anything like how you come across in this thread, he'll either just laugh, happy slap you or lock you in a tiny tiny box, filled with woodlouse.
How about you just leave it?
Agreed. If you try to come across as something you're not, then you will do more harm than good. Are you really that good an actor? I doubt it if the notion of violence makes you sick.
I would tell your mother that you have other things to concentrate on, and that you're not the right person to do the job. Suggest that she takes the video to the school, where they will have a disciplinary system in place to deal with things like this. She will (hopefully) respect you for your maturity.

