This is quite long - sorry.
We had something similar with our little boy in the autumn term of P1 (we're in Scotland). At this point he was at a small independent school close to my work. He moved up from the school's own nursery the month of his 5th birthday (August baby) with no doubts raised about his readiness for real school. The original P1 teacher lasted less than a month, and went on long-term sick leave. Two supply teachers over the next 6 weeks, then a long term supply was found.
All through this, his reading was coming on in leaps and bounds, as was his maths. Homework however was a nightmare; it was daily Jolly Phonics worksheets which involved writing letters and colouring in a picture.
But all through this his behaviour at home was getting worse and worse and we were getting more and more concerned about him. But there were no issues raised by the school - his first terms report, in early November was fine. I dropped him off in the classroom every morning and my husband picked him up every night - no issues raised.
Two weeks after the report, we had a parents evening at which we were told that his writing and concentration were appalling and they were seriously worried about his behaviour. Even told me his table manners were dreadful (he was 5!). Oh, and he was hopeless at PE. We were summoned in to the school twice more that term to discuss his poor behaviour.
Turns out, he has very mild dispraxia - atypical because his speech and vocabulary are well ahead of his age. His teacher simply could not grasp that a child of 5 could be confidently polysyllabic verbally and barely able to hold a pencil - so she assumed he was being difficult. I spent the next 2 years trying to keep the same bloody woman off his back. We eventually moved him after 2 years to a middle of the road local state school, who identified the problem. Of course, because he is bright, no help has been forthcoming because he is 'coping'. By this time spelling is also a major struggle.
So in the autumn we are moving him again, to another independent school, which has a stellar reputation for bringing round children whose early experience of school has been poor, and for getting the best out of boys. We were completely open with the head, our son has had an assortment of tests and they seem quite prepared to cope and confident that they can help.
My Dad had similar problems as a child (probably more severe) and thrived once out of the school environment. My husband also hated primary school - and thrived once he made it into the higher levels of secondary school. I had similar physical problems, but being female and very academic, managed to cope (except for Art and PE, anyhow)
Anyway, to get to the point, finally, if your son is suddenly behaving in a way that is completely out of character, try and find out if he is being expected to do things that he is simply not yet ready for. And if that is what is happening, do what you have to to get the teacher to back off unless she is coming up with specific ploys to help him.
And a more general point - why have we allowed our schools to get to a point that so many little boys have an absolutely miserable time at an age where it really doesn't matter what they do so long as they are learning something. If I had the time again, I would have kept my son in nursery for another year - and found another school much sooner.