Thank you Benjimoon that was really helpful.
Now, I am going to impart some of my thoughts, which are of course not something anyone has to read or agree with.
I have realised that I feel defensive about DS. What makes us good mothers does not necessarily lead to clear thinking. Despite all those posters who seem determined to believe he is a PITA (whatever that is - I've yet to see a definition and yes I do know it stands for pain in the arse) and that he might even have been the author of his own bullying, I can clarify the latter is definitely not true. I've seen, as has everyone else involved, the evidence. Screen dump, after screen dump, after screen dump - reams of pages of it. All there for everyone to see. No doubt who was involved, who the protagonists were, who stood by, who knew etc. The issue I'm trying to get to the bottom of now, is how it went on for so long and address all the failures of school policy that should have protected everyone along the way.
Most of my work involves relationships that have broken down one way or another. I'm usually trying to establish what the law is in those cases. A great deal of the time, the reason things have gone wrong is because of "assumptions". Humans are as clever as they are because they assume things and they are as stupid as they are because they assume things.
One popular assumption is that "everyone does it" - which is something that has appeared in this thread a fair bit about bitching about parents & pupils. In my experience, everyone doesn't do it at all ever and thinking that everyone does leads people to make bad choices.
Another popular assumption that gets people into bad situations, is that they think they know what other people's motivations are and what they are feeling. Again this has happened a number of times in this thread. But, I'm also doing it trying to work out why the Head of 6th seems to dislike DS and why the other teacher sent the rude email. It is a pointless exercise and we're wrong as often as we are right, so I shouldn't be doing it. I will probably never know and it actually doesn't matter.
I'm going to say again that my experience leads me to believe that gossip, tittle tattle and bitching is not a good thing to do. A bit of Adlerian psychology would suggest that the perpetrator's goals
when they do it, is for themselves. They are seeking a gratifying reaction, be that sympathy for having to deal with a PITA or admiration for knowledge passed on etc. My experience leads me to believe that when information is passed on that way, it is often exaggerated to ensure that the right response is garnered. After all if you say, "I found myself mildly irritated by John Smith in class today", that's probably going to illicit a bit of a meh reaction, but if you say "OMG, bloody John Smith was a real PITA in class today", you're likely to get the response your are actually seeking. I see this a great deal and the snowballing effect it has, that again leads people to resort to courses of actions that they shouldn't and were actually based on mostly false information.
We also project, which again is what makes humans clever but again stupid too! We take previously encountered scenarios and based on vague similarities, project what is likely to happen. With driving a car or cooking, that is very helpful, when we are dealing with other people, it isn't always the case. Lots of projection here on this thread about the type of pupil DS is and the type of parent I am, based on previous interactions with other pupils and parents - but with remarkably few facts to base those views on.
This was a AIBU and WWYD, so I asked for it and I got it and I'm glad I did and there hasn't been a single post that I haven't taken something away from.
I don't wish bad things on the teacher who wrote the email but I also don't want her to have much to do with me or my DS going forwards or the head of 6th either for that matter.
I also want to seek address and better ways forward with reflection on what went wrong with the bullying and how the school can either tighten their policies or fully apply their policies as a result. DS is fine but other pupils and parents should have some degree of confidence that the school knows what it is doing with it's own policies, even if all the aims, values and Christian principles are guff (technical term!). However, if they are not going to do this, in what I would see as the easy way, I do now have some leverage and pointers from this thread to do it in a more formal way.
I don't think I can really add much more now but thank you all again.