Well, I have 3 children, 8,5&2.5, and I haven't potty trained any of them, and fwiw have no intention (at least atm) of starting any time soon with the youngest.
We tried training my 8 year old at just over 2yrs but my FIL became terminally ill and we were up and down the motorway every 5mins so we put it on the back burner. Then my second dd was born. Her 3rd birthday was approaching and I was well aware that most of her peers were dry by day and tbh I was starting to think I'd missed the window. Low and behold, a couple of days after her third birthday party, I was about to go out for my weekly shop and asked her to get her nappy on quick. She refused point blank, told me she was a big girl now and wore nik niks. I grabbed a bag of spare clothes and that was it. From that point she only EVER had 2 accidents - and that was including night time. She had loads more when we were "training" her (like a dog!!)
My second dd was just over 2 when I asked my elder dd to demonstrate on a potty (she was 4 at the time and found it hilarious). She had a few more accidents but not many.
My youngest will be 3 in Oct - he will use his potty in the house if he isn't wearing anything - put him in underpants and he wets himself. He has recently started taking his nappy off for number 2's. Honestly, I'm in no rush - nor am I lazy thank you very much. I have every faith that he will get there in his own time. Yes, "when he is ready"! Why put yourself through a week or more of stress, which may result in them being mostly dry but with their fair share of accidents?
There are very few children who start school in nappies - and even if they do it's not long term and are even more unlikely to be wearing them as teenagers. It's a developmental stage.
If someone was refusing to let their child stop wearing nappies, that, imo, is a completely different matter.
JMO, but I feel schools should have some written policy on this (whatever way) which is made available to parents before their children start school, and parents should have the opportunity to discuss these concerns well in advance.