I don't think you're over-reacting at all. I think everyone would see immediately that it is inappropriate to proselytise to children in this way, without the parents being forewarned, if the religion being preached was a different religion like Islam or Hinduism. But because it's familiar old Christianity it somehow manages to wiggle its way in and people are much less inclined to question its right to be doing that.

Evangelical Christianity is more worrying to me than the usual C of E stuff. I think you'd be right to complain about the way this was done to the school. I don't approve of schools sending their children off to organisations which are going to try to indoctrinate them in the first place but if they're going to do that they should have at the very least been up front about the organisation.