gosh, what a nightmare. I guess at least she asked, and she cares about your children, that at least is positive. But wow, way to overstep boundaries. A short, sharp, "no" will hopefully suffice, and re the school: it shouldn't matter who volunteers, YOU are her mum. Write to the school and state that no-one except you and her dad are to be contacted regarding school issues, and that both of you are to be informed of everything.