First up, I haven't read the previous thread so apologies if this has been said before.
My DS taught himself to read age 2. We had no idea until nursery called us to ask how long he'd been able to do it. Within a week they asked us to send in books for him as theirs weren't challenging enough. They then decided to see what he was like with numbers. His nursery teacher was fascinated by how quickly he learned and by the time he left for reception he was able to read pretty much anything (and comprehend it), would write his own little stories and was doing basic addition and subtraction. Of course in early years they have very high staff to child ratios
He then started school where he was pretty much left entirely to his own devices for the next 2 years. At the first parents' evening the teacher was very defensive and basically told us that she was aware of what DS could do but we needed to understand that the rest of the children couldn't so she would be devoting her attention on getting them to 'catch up'. DS spent most of his time reading quietly in the corner having finished his class work quickly. Most of his maths lessons were spent reading because he'd get through the class work quickly. We asked for extension work - or for them to give him extra work on his weaker areas (terrible handwriting) if he finished class work early. They weren't keen. It seemed as if they were actively trying to slow his progress and they repeatedly told us it was only a temporary problem as the other children would catch up after a few years.
DS is luckily easy going and was still happy at school - he is always happy if he has a book - but it was apparent to us that they viewed his abilities as more of pain than a talent.
This was at a private prep so we did the 7+ and moved him for the start of year 3. The difference is unbelievable and he is so much more enthused and stimulated. Before, if we asked about school we'd hear about lunch and playtime he now comes home full of he discoveries they've made in science or the new methods they are learning in maths.
What is interesting is that it isn't because he is now only surrounded by super able pupils (although there are definitely more than average) - the school's main intake was at 4 so selection was limited to a short interview apart from the boys who joined later. His class has a huge range of abilities and quite a few boy with SEN. The real difference is that his new teachers aren't working towards some 'average' level for the children's age. Instead the focus is on each boy doing the absolute best he can. They are all given individual personalised targets for each subject, given whatever support they need and praised when they reach them.
It's about looking at each child as an individual and it's something that the private sector can be just as bad at as state schools. I suspect because school management, by necessity, judge a teacher and a class' success by looking at the average performance, that is where most teachers devote their time.