To all those parents being a bit bolshy about it "not being legal", schools do only do this for the good of your child, you know. It's not like nursery where babies and toddlers start at different times all year round. Having 30 brand new children start brand new routines with a brand new teacher, in a brand new environment - all at the same time - can be VERY difficult for little ones to cope with. If a couple are crying this can often spread quickly to others and the teacher can't give their individual attention to every crying child at the same time. It takes time for the teacher to get to know each and every child and the best way to deal with them when they get upset as all children are different.
It's quite hard to do this when there are 30 in all at once, and some with obvious additional needs or behavioural issues which parents kept quiet about on the application form. Then there are the ones who aren't yet potty trained and who need cleaning up after an accident.
Try to imagine a Reception school classroom at the beginning of September with 30 children in from day 1. It's easy to imagine - have you ever hosted a kids' party? Now imagine that party with 30 kids, who you don't know, don't even know the names of, and doing it on your own or with maybe just 1 other helper? Not only that but the party is 6 hours long, and you're not able to call the parents to come back early if they child is crying and various children are wetting themselves or spilling food down themselves. A few are being naughty, not listening to a word you're saying. Others are screaming as they're non-verbal or have suspected ASD and they can't cope with it all. Maybe one is shoving someone, and another one is hitting someone. You have 15 children in your face all trying to tell you something all at once.
Be VERY grateful that your child's teachers do their best to take on this challenge as best they can from about a week in. The chaos and upset to the children is minimised by having a gentler run-in for them. They're not thinking about whether you're annoyed because you've already got your annual leave earmarked for other things or if your manager is being difficult about you needing to adjust your day for a couple of weeks. They only want the children to settle as quickly and happily as possible. You've known that your child will be starting school around that time ever since they were born, it doesn't come as a surprise to anyone, so there is plenty of time to plan ahead to think about how you are going to manage the slight disruption for just a few days at the very start of the next 12 or 14 years of their lives.
If you have never been in a Reception classroom for any length of time I think you'd find it an eye opener. Those teachers deserve a medal and a stiff drink at the end of each day. 😆