I was able to speak to the teacher, and its actually worse than I thought.
The year 2's are being told their letter is to the real santa and it has already been posted and they will get a reply.
The year 4's are being told the letters they receive are from santa and he wants help replying to them, and they are recruited as elves for this purpose. They are also told the letters are all from the year 2's at their school 
The year 4's are not replying directly to the year 2's though. This is the best part. The freaky santa head which the year 2's are being told is actually santa is going to be used. The year 4's will record verbally their reply to the letter they were given and it will be put into the freaky santa head computer which will speak the childs reply but in a 'disguised' voice. The year 2's and 4's will get together in pairs so the one who wrote the letter and the one who replied, to watch the freaky santa head reply.
You just can't make it up!
The year 4's are definitely therefore going to know that it isn't santa and honestly so will the year 2's because the santa head is like a thing from nightmares. Also the year 4's will have to keep it a secret that its not santa and its not them who replied. So basically lie to the younger ones. WTF
The santa letters will not be sent home before all this happens, but will be sent home at the end of the term (a bit fucking late, i'd say).
I said I wasn't happy about it. I told her I'm not happy with my child being shown a nightmare head version of santa and being told its real. I should be the one to choose if my child goes to see santa and choose which santa that is and not have a teacher tell my child this head on the screen is actually santa.
Santa in our house is magical. And rarely seen. This is far from it.
Also I told her writing the letters great, but I have no idea what is on my childs letter and even if the year 4's are not promising them they will get whats on there, they are given the impression that if they are good santa will do his best.
That to me is going to translate to a 6YO that they are expecting to get whats on there. Teacher couldn't see issue with this (she doesn't have DC which I think is very relevant here as clearly has no clue parents need a degree of control over this).
Her argument was it is for the children whose parents don't do santa letters with them. Which would be a small minority.
I pointed out that the parents who don't bother with Santa letters, possibly may not bother with presents either, or don't do christmas and the child could actually end up even more sad and dissapointed because they actually wrote a letter to santa, got a reply and then on xmas day, nothing.
Surely thats worse than not sending a letter in the first place? And those that do buy presents but don't do a letter, so what? The child is not missing out.
I was a bit cross actually, but think I kept reasonable.
I have requested my child not participate at all in the whole reply, video, lie thing they have going on and to send the letter home and I will arrange a reply to it (once I've actually seen the damn thing). And because they have already promised my child a reply.
A few other parents have messaged in our group chat thing and think the same, so there won't be just my child not participating in it.
I'm happy for them to have letter writing experience. But as one PP said, they can do pretend pen pals for that. Or do a test run of their santa letter.
But I will have to explain to my child why they aren't doing it, although I've been told not to tell them its not real and just for school in case they tell their friends!
A friend I have with a dc in year 4 is livid. I really think they are going to have to pull this one back.
What the hell is wrong with making snowflakes and christmas cards, or nativity scenes or something? Or write a letter to an old person, wishing them a merry christmas or something.