If they did that to my child, I’d put those tattoo instruments in a rather unpleasant place.
You probably should spend a few minutes on Google before sharing your ignorance publicly. No 'tatoo instruments' are used in a henna tattoo. They are either painted on with a stencil or freehand, and they last anywhere from two weeks to two months.
At least here, the school gave the parents advanced notice of the planned activity and the parents didn't opt out.