For me, it was because I did flirt with the idea of sending dd to school. After 15 years of home edding, when dd got to 5 I decided I was tired and needed a break, and so she could give school a try.
She went along to one taster session. We had missed the first introductory session where the parents stayed as we were on holiday, so as it was all new to dd she asked me to stay with her, which made me the only parent there. The teachers were not too cool with this, they had planned this to be a serious sorting out session. They had got a page of the alphabet out and had the children in a queue, one by one quizzing them on what letters they knew. Dd loved it! She whizzed through the alphabet naming every letter, but some children looked clearly ill at ease, bored with the queuing and didn't have the first idea of what to say. They were already beginning to have an idea of what it was to fail The teacher corrected my dd on one or two of the letters, telling her she was pronouncing them wrong - "its not nuh, its nnnnnnn". I was a little put out that they didn't tell her that some people said it that way, but they prefer another, instead of flatly telling her she was wrong, but ok, I could live with that.
Dd is lively and buzzed around the teacher asking what she should do now - in order to keep her occupied the teacher gave her a piece of paper and told her to write down 1 - 10. Dd wrote her numbers, but got one or two backwards way round. The teacher started to tell her they were wrong, but I intervened and said that at 4 I thought it was pretty cool she could write any, and I didn't think it was a good idea to point out problems as she would self correct as she saw more of the numbers. The teacher unwillingly agreed.
Dd still looking for entertainment found a computer covered over and asked if she could use it. A teaching assistant was set to uncover it and find her a simple art program to use (far simpler than the programmes she was accustomed to use at home). At this time I was shuffled off with a patronising, "mummy is tired now, let mummy go home for a rest and come back for you later".
When I went back for her she was carrying a picture of a tree, but I was a bit mystified that she didn't seem very interested in it. She said she had had a lovely time at school, but she couldn't do numbers. The teacher had taken the opportunity to go back to her and explain her numbers were the wrong way round - however my dd took this as not being able to do numbers and this has stayed with her ever since
I asked if she wanted to tell me about her picture, but she said "oh, I didn't do it, the lady did it". So.....they had her for a couple of hours, taught her that she couldn't do numbers and her attempts at art needed an adult to intervene and make them "right".
She didn't ever go back