Self care is restricted to dressing, though. It doesn't include any other important aspects of self care. You have no idea if the children are being taught those other aspects at home or getting the chance to practice their dressing skills either. Dressing and undressing are only a small part of the self care picture.
Some children do not have the manual dexterity needed for buttons, some are unable to tie laces because of a disability. Putting velcro into a shirt or wearing velcro shoes speeds up the changing process, but they are adaptations, not skill-learning -- a lot of parents of SN children resort to this sort of adaptation. DCs' school recommended velcro shoes for all the younger grades even before the change in the uniform policy, because they made things easier for everyone, students and teachers alike.
Not every child with SN has the same needs, and how do teachers keep track of who needs exactly what in terms of instruction in self care? I suspect there's a lot of helping and not so much actual teaching or training, given the time constraints and the number of children involved. If you have a school where there's a significant number of children with SNs or who can't dress themselves for whatever reason, all the more reason to waive the changing requirement.
Having the children all change together at the same time just provides an opportunity for some to feel smug and superior, to tease the slower or clumsier ones or those who obviously have mummy do most of the dressing at home ("babies"), both verbally and by hiding or stealing gear. It's an occasion where embarrassment is guaranteed for some children.
Are schools worried that the majority of children will grow to adulthood and still be unable to dress themselves unless they intervene at an early age to save them from their lazy parents? Most children have the ability and the desire to dress themselves and do self care by age 8 at the latest.