I agree with all of this, except that letting the meltdown happen can be dangerous in a classroom setting. What if he progresses to self-harm? Most classrooms have plenty of things to bang your head on, intentionally or unintentionally. Or sharp corners to run into, etc. etc. And what if he progresses to harming the OP? Sure, a teacher could generally fend off a 5th grader....but if they're not trained in aggression control techniques, one or both could really get hurt. And the sad reality is, if the child ends up hurt, even if he does it to himself, the teacher is going to be held liable.
That's why we use restraints in psychiatric facilities. It's not punitive. It's to literally stop the person from being injured. And restraints, especially for children, are more likely to be physical holds rather than mechanical. A basket hold is particularly useful for a child who's melting down, because it provides physical comfort as well as preventing much movement. But a classroom teacher isn't trained to safely perform one.
I'm just coming at this from the perspective of someone who, at various points in my residential psych career, has been stabbed with a pencil, spit on, kicked, punched....one time an adult male in a transitional facility got hold of a kitchen knife. First he slashed his wrists, then he came at staff. Took three of us to get him into a takedown while someone else called for the hospital to pick him up. We all ended up bloody and the facility director was nearly strangled with his own necktie. My ex-husband had three fingers broken when he was restraining a girl who was violently bashing her head into the floor, so he put his hand under her head to prevent a concussion. My dad got two ribs cracked when he had to restrain someone alone on the overnight shift.
But we trained for it. My first aggression control class was taught by a VERY large and strong man. For the final exam, myself and another girl, both quite tiny, had to disarm him (fake gun), take him to the floor, and get him fully restrained, all without causing him any sort of injury, while he was fighting full out. The goal, always, is to protect the person who is having the meltdown. Because like you mentioned, at that point they're not rational. They're not feeling normal pain, and they have to get it out before they can get back under control. This is way outside the average classroom teacher's wheelhouse.