This letter was not sent out in error, as the head would have read it and been happy for her signature to be there, so this is just a bit of back tracking when they have realised that they have pushed the racial boat just a bit too far.
I am not sure whether I would have let my children go on such a visit in the first place, but then I would have looked at what sort of visit and religions were to be looked at before making my decision as to whether to allow or not. And before someone shouts racist, I am far from it, as I am agnostic and have seen/ experienced many faiths, and there are those that I would not wish my children to experience. That does not make me a racist, just a caring parent, as I have seen Islam at its best and worst out in the middle east.
I would certainly have taken the school to task over the letter had I received one such, and would certainly seek to have the head disciplined over a threatened abuse of power. Imagine threatening to leave something against a child's name to carry all the way through school because of the action of a parent. Such nonsense leaves the school and authority wide open to litigation and shows the head teacher to be truly irresponsible.
Imagine the other side of the coin, where there was a strict Islamic family (only using this as an example, I have nothing against Islam but think on this, the prophet Mohammed married a 6 year old then consummated at 9, so be carful what you want your children to learn), back to the family, and a school was going to take children on a visit to a number of churches both Catholic and Protestant. Do you think for one second if the Islamic family removed their child from such a visit there would be a letter such as this? I think not. If such a thing happened, the head teacher would be removed from their position, so why is it ok the other way round?