Sorry, that I am so late to this table OP, and I am exhausted as I haven't been able to sleep yet, therefore I have unfortunately only read a small amount of this post, . But as I absolutely hate hunting I completely agree with the stance you took.
(If I were healthy and able to walk, I would be a hunt saboteur - as long as no dogs or horses were put at more risk by the sabotage than they would have been in taking part in the hunt in the first place.)
I think it is very likely that the governor who wrote you a letter is actually a hunt member, and probably of that particular hunt (maybe he even suggested it in the first place?) I also think that most of the posters who are responding to you negatively, are at the very least hunt supporters themselves.
No-one who cares about foxes being terrified into running for their lives, and being torn to pieces by the hounds when caught, or who cares about the fate of the dogs once considered past their useful hunting lives - they are usually not deemed safe enough to become domestic dogs, and certainly not for families with children or other pets, or who have other animals like cats coming into their gardens - would disagree with you here OP.
I am sure there must be something like a petting farm within travelling distance (and one that doesn't openly support any hunts), that they could visit instead, or some other nearby educational attraction that does not include being cruel to animals as one of its activities.
When I was at junior school (many years ago admittedly) I enjoyed a visit to a main post office sorting office, The Natural History Museum, the remains of a Roman Settlement, The computer room at what was then a local polytechnic - it was literally "the computer room", with one enormous computer in it, that printed out our names in Ticker Tape! Now I know that last one is very unlikely to still be found (except in maybe the Science Museum in London, which is of course another fascinating visit for anyone who lives near enough).
We also visited an amazing Planetarium, I know there is a smallish one in Bristol's Science Museum, and of course the brilliant one beside Madame Tussaud's in London, but hopefully there are more dotted around the place - I know that Yorkshire has some great museums, particularly in York, but in other parts of Yorkshire as well. We have so much heritage in the UK, but we don't seem to tend to make the most of it.
So I am sure that wherever you are OP, that the school could manage to find somewhere else to visit, or even take picnics and do a nature ramble, which could include bark rubbing, leaf identifying and drawing in a sketch pad, lifting big stones to look for insects underneath - maybe one of the teachers or volunteers could bring a portable insect viewing telescope, and the children should be taught how to gently pick up an insect, and then to return it safely to the same place when finished with.
I had better stop now, but that last one is almost free, and doesn't need booking in advance unless they are going to go a little further afield - excuse the pun please - which they would still need to book a coach for, but a lot of children enjoy a coach ride and might appreciate a small change of scenery. So what I am trying to say in a round about way OP, is that I don't think anyone has a legitimate claim that you have caused the children to miss out on an interesting and fun day out, they can still do that, as there are plenty of choices, even out in the depths of the British wilderness!