@workworkworkugh
So the school has contacted me and apologised profusely for what's happened.
I've spoken to the teacher that we've had the most contact with and who has been helping DS with a few other things (one part of the email was directed at her and was quite rude).
She has told me that she actually had a meeting with GF (before this email was sent) and did ask her a question about DS, apparently GF became hysterical and angry and basically said how dare the teacher even ask her anything to do with DS 
The teacher pointed out to me that she wondered if it was because she was another female/mother figure in DS life who he was turning to for support instead of GF and she's feeling threatened.
Spot on if you ask me. Probably why she targeted this teacher in the email.
The school is going to be speaking to GF about the email and the inappropriateness and also talk about boundaries.
She's starting to come undone and it's all her own doing. There's no one else to blame. She's scrambling to keep control. God help us all when she realises how much control she's losing.
That’s not good enough from the school - an apology and talking to the GF about boundaries?
They are keeping this at a v low key level for their own self interest and not in the interests of your DS trapper in an emotionally abusive relationship.
This is a specific moment where if you escalate this by stealth you can potentially have a very significant shift in dynamics for your DS.
This needs to go to Sr Management - it’s a serious breach of protocol and a huge safeguarding risk. They are minimising it for their own ends. They need to call in safeguarding experts in on this - it’s above the pay grade of the class teacher and counsellor.
Your DS should never know that you are behind this but this blows open the whole dynamic to be exposed in another setting.
As PP have said this could give him the ladder to climb down to save face as it’s other “neutral” voices who could talk him.
They need to be involved to see the trajectory of how he has failed / stalled / regressed academically, socially, in sport, with family relationships through the abusive, controlling and isolating threats of this girl.
The school need to be fully aware of what he has endured and offer a safe and supportive space for him to find himself again.
Don’t go near the dysfunctional parents of the GF - they cannot see it - their permissive, unboundaried, indulgent, lazy parenting is what has enabled her entitled, deluded and malicious behaviours.
I think that you are v astute and correct that the flip out at the teacher is a pattern of feeling under threat from another female of influence - and that’s exactly why this needs escalating above this teacher to someone with skill and insight because the GF will do exactly as she has done before and alienate your DS from his teacher.
Keep calm. Escalate with facts.