My DS (also on the spectrum) finally cracked dressing himself in Year 4 (8/9 years old). Before that he would need help from the TA as he was slow, easily distracted, had poor motor skills and would have probably been quite happy to do the rest of the day in just his pants! He was holding up the class from moving on to the next subject, so with my permission they decided to have the TA keep him on track, remind him what came next and how to turn his clothes the right way.
At home I let him dress independently, but then we stood in front of the mirror and checked ourselves and each other (I'd sometimes put something on inside out or back to front for effect). I was doing all the same stuff as school but it took him much longer to grasp. When he first started dressing himself at 3 he would just pit his clothes on over his pyjamas. He still occasionally forgets and starts to do this. He also needs prompts to dress appropriately for the weather, otherwise he'll wear a vest, t shirt and heavy hoodie in the height of summer.
He's 11 now, and the other day I overheard him say crossly "Oh FFS!" (Yes he did use the letters only!). I called out if he was ok and he replied, "I'm so rubbish sometimes, Mum, it's just taken me three tries to get my shorts on right!" We laughed about it, as I had the same experience with a dress last week.
I think OP is overreacting by being furious, but the school could offer him more guidance so he grasps things quicker. It does take children with SEN a lot longer to master these skills, and they usually need strategies to help with this. And the strategies need to be used at school and home. Kids with SEN can't just learn a new skill over a weekend at home. It takesmonths of repetition. If it were as easy as some posters suggest, I'd have an 11 year old who could tie his shoelaces.