Re the idea that every kid has trauma, and every adult had it as a kid too therefore it's kind of irrelevant, that misses the point.
Trauma in this sense is not seen as "big T Ttauma" like rape or heavy domestic violence. It's "little t trauma". This is better viewed as unhealthy stress.
If we look at what healthy levels of stress are for kids (NOT what are normal levels these days) then kids today do have a lot more stress in their lives than before. A lot more than the average level we did. The ACES list events in children's lives that add significantly more stress (as well as "big T events"). These are noted because they are SHOWN to have a negative impact on the child, and later the physical and mental health of the adult.
Think of this scenario. You're having relationship difficulties with your spouse. Your mother has just been diagnosed with dementia. You're not sure if you can afford even a small holiday in the summer and your colleagues are all going somewhere. You see Facebook photos from friends having a great time at another friend's birthday party, that you hadn't known about. Not one of these is a big trauma, but all add to your stress levels. Now add in that your work expects you to do a one training course at the end of which are exams. If you pass, you'll get a great promotion. If you don't, you'll never progress in your career. You turn up in the first day if the course and you've forgotten your pen. That's a mark against you already. You finish the first session and have a joke in the corridor on the way to the next one and you get another mark against you for that. The next session begins..and you're in trouble again for forgetting your pen.
Would you honestly be feeling relaxed and ready to learn in that scenario?
Some adults in exactly that situation would never forget their pen again. Others would forget, others would be without one until payday. Some would figure something out and be fine. Some who always had their equipment, who never got into trouble would be running scared of being in trouble. Others would just wonder what the point us.
Clear and fair expectations for children to meet are not unreasonable. And things like posture are important to low levels of mental health (ever sen someone really happy walking with their feet trailing in the ground and a grumpy face? It's not common for a reason).
But if they're punitively enforced, they add further to the stress. They unfairly impact those who have stressful lives outside school.
If we look at the private schools who hothouse children, they almost always have a strong sport programme. This actually helps reduce stress as intensive physical activity changes biochemistry. Now, it's not that sport alone is the answer, or that teachers need to pander to the emotional needs of kids. If, however, children are being treated like they are all the same, that they should be able to behave, because others are, then the system is actively being set up to advantage those who already have certain emotional advantages (safe, secure, stable loving family who live harmoniously, no financial worries, secure friendship groups, being accepted for who they are etc - OR the odd few who can "work their way out of poverty", as if it's hard work that reduces poverty..). It's not about kids who live in fancy houses being ok, it's about those whose emotional health is undermined having it further compromised by a school system that treats them as bad for reasons that are often stem from reasons without their immediate control.