Ask to speak to the schools SENCO/inclusions manager. State that you have a severe concern about safeguarding and that you are worried about your childs mental health. Ask to see their policies on clothing not related to the school, to bullying, to the use of camera's and or phones during lesson time etc. Ask how this behaviour fits into this ethos.
Let them you know you want to know what action will be taken from this, and if it is not sufficient you want to meet with the governors and will be letting either the LEA or the academy chain know as well as speaking to the government.
He should lose his job for that. It is not just unprofessional it is borderline child abuse. It will also be worth asking what happens to the film of what has been taken, and stating that you wish for that to be destroyed, and you have not convened to your child being filmed while on school property. (It may even be worth asking to see what their GDPR policies are).
You have a strong case their mate. The fella should lose his job, you should receive a formal apology and potentially compensatory damages. The lads filming it may even be excluded with a bit of luck (and thus ruining their GCSE chances which would be a nice bonus). Any teacher/school manager refusing to act upon this could also end up losing their job.
I'd also suggest tentatively sounding out a solicitor as well.
If you want more advice don't hesitate to DM me and I can elaborate on how I would move forward with each stage. I'm sure there are some legal people on here who I am sure will be willing to help. However be very certain, that an adult on Locus Parentis who is harming and violating a child under their care, and then allowing the abuse to be filmed without the consent of either their child, or their parents will be in serious trouble. I suspect only a handful of (noncey) Kopites will justify this behaviour as "banter". It is child abuse.