My ds was fine in infants, but went into freefall when he went into year 3 and nearly had a breakdown, even though he's at a primary and they don't make a big thing of the transition to juniors. It was heartbreaking to see him change from a happy confident child into an anxious, stressed shadow of his former self, over the course of just a few weeks. 
The things that bothered him were a greater need to be self-organised, remember things on his own and work undirected for periods of time, having several teachers rather than just his class teacher and a TA, peer group relationships changed a lot and the others in his class seemed to mature quite a bit over the summer holidays, leaving him obviously behind them socially.
He had his assessment in January this year (ADOS) at age 8.5, following intervention from the Ed Psych and ASD Inclusion Team and was diagnosed with Aspergers.
He came out of infants top of his year across the board, but has made zero progress since, so he was a level 3 at the end of year 2 and he's still a level 3 (now in year 4, just about to go into year 5).
Academically things have changed a lot since infants, maths and literacy have become more complex and require transfer of skills from one lesson to the next etc. Literacy is also requiring a greater level of comprehension of what is 'unwritten', so abstract and lateral thinking is coming into play and ds is really struggling with that. He is still pretty much top in mental maths, but barely writes anything during maths lessons, as he panics at multi-level problems and can't get started. As a result his self-esteem has plummetted (sp?) and he's given up trying.
Juniors is definitely a completely different ball game socially, my ds has one really good friend who is an angel and has stuck by him through thick and thin, if it wasn't for him, I don't think ds would still be in school.
That said, now the other boys are also interested in computer games (ds's specialist subject
) he has more in common with them socially and seems to be doing a little better and mixing more. Of course this is also post intervention and following a lot of hard work on his social skills. He is still markedly delayed socially compared to his peers and gets upset very easily, which again marks him out as different, but for the most part the children in his class are very tolerant and supportive of him, despite not actually knowing he has AS. Actually, his outward bound residential was a positive turning point for him socially, as not only did he find it empowering to try doing some things he was afraid of, it gave him longer with his peer group and enabled him to find common ground with some of them.
In my experience, he seems to go in peaks and troughs of coping and not coping, pretty much depending on how regular the school routine is and what's going on in the playground.