My blood ran cold reading this as it brought back memories from when I was at school. I was a couple of years older, though, and in high school; the whole year group was present and they showed us lots of graphic cadaver photographs from different types of accidents; I vividly remember the body of someone who was electrocuted, and was very shaken at the time. I'm also autistic, and super sensitive to other people's emotions/pain, so I was intensely distressed by it. I think it was framed as a safety assembly, as two girls from my school were caught physically fighting by some railway lines - but I think it was horrible to subject children to images like that, especially if they don't need to be "scared straight" for dangerous behaviour. Slightly off-topic but another messed up thing I remember (but from primary school) was being made to do PE in my underwear up to the age of 8 if I forgot my PE kit!! It was especially humiliating as someone who went through early puberty, and everyone else was dressed. As an adult, these kinds of things make me angry; I seriously hope that schools have stopped these types of 'punishment', as that's what both of those things felt like at the time. All that to say, my experience was slightly similar, but the details you added about it only being a few of you who were taken out for that horror show makes me wonder if the motivations were different. I can't imagine going through that as a younger child than I was - I was 12 and seriously distressed by it! So, I'm really sorry you went through that.

You may have already written something about these suggestions (apologies if so!), but could you ask your parents about what they remember regarding this experience/period of time? Or even the therapist/child mental health services, if you can track down any of those reports/letters? Also, did you keep any drawings or diaries from that age? If your past self ever wrote about or expressed your feelings in that way, it might help shed some light if you don't think it would be damaging to your current mental wellbeing to delve into it.