What a relief to read this thread! My DS2 has just finished Reception and has been in trouble an awful lot. Like your DS, mymama, not for nasty behaviour but more for immaturity and silliness. He's worse at playtime than during class time. He doesn't seem to be able to play well with other children, either.
His teacher has also singled him out in front of the whole class. They do a lot of partner work and instead of working as a partner, the teacher said that someone had to volunteer to work with him and that child would be the "leader" instead of the "partner" and DS2 had to do as they said. I think it's part of this Co-operative learning technique they use (but could have totally got the wrong end of the stick!).
DS1 had the same teacher last year and did very well, but he is very conformist and, I think, easy to teach. It was a real eye-opener to see the same teacher with such a different child.
What did improve DS2's behaviour was when, on the occasions his teacher spoke to me about him, I strongly reinforced the way he was expected to behave. He responds quite well to being given a list of 3 rules. In our case, we had: focus, follow the school code of conduct, look at the speaker. It wouldn't work for every child, certainly not for DS1, but it made a big difference with DS2. We had a spell when the teacher would just give me a glance at pick-up time, or a thumbs-up or down, so I knew what sort of reinforcement was needed at home.
I intend to speak to his year 1 teacher in September and ask if we can set up a similar line of communication - that doesn't involve me hogging her time but enables me to work with her.
Mymama, what is your DS's year 1 teacher like? Will he/she be a contrast to the reception teacher? He sounds very like my DS2: essentially a "good boy" who would love to be praised and rewarded but can't quite marry up his behaviour with consequences!
DS2's reception teacher is very authoritative, quite controlling, pretty firm - she's very kind with them but tough as well. The year 1 teacher is a wispy little thing who many of the parents find it difficult to warm to, but apparently the children adore her. I'm hoping she'll make DS2 feel more confident and special.