Getting one of mine to school was like stuffing an octopus into a string bag and then getting an octopus out of a string bag.
First one had to extract them from their pyjamas and stuff them into a uniform, then extract them from their bed/room/flat. I've carried a thirty kilo "octopus" down three flights of stairs peeling fingers and feet off the architrave, the banisters, and anything else they can attach to on the way down.
Now they are bigger I have to rely on persuasion and working with the school. Kiddo went "missing" recently, which has helped get them more support getting into school.
My tips:
Keep calm. (!!!) (Sing-songy or soothing voice, normal voice=anger to them even if not true)
Reduce the demands in the morning. (Or do as much of their getting ready as you can, maybe dress them) (your aim is to get them into school as ready to learn as you can, life skills can come in the holidays)
Remember you are not choosing options from the ideal. (eg drink ideally would be milk or water, but for your kid it might be nothing or coke and in hot weather when they won't drink in school to the point they feel ill, it's coke)
Work out your non negotiables and what is not important.
If they are old enough for a phone, ringing them up/texting to remind them to get dressed.
Work with school to have a back up plan: they could gomin early or late or through the office for example.
Stop caring about stuff you can't control. (Eg if your options are between being late or not going in at all stop worrying about being late, but do work with school on that. You can work with educational welfare as well to get them to help you put in reasonable adjustments at school)