Hi - exTA (male) here:
I have worked in very small schools, where YR, Yr1 and Yr2 were all in the same room with one teacher and 1 or 2 TAs. It worked fine, because the teacher was very good, and differentiated work according to child's age and ability. YR mostly did 'learning through play' activities, but one or two of them with more academic interests would sit-in on numeracy or literacy sessions.
Schools vary so much that it is impossible to compare them, or to generalise: this is very unfortunate, as a standardised curriculum and Ofsted monitoring should have reduced these inequalities, but this seems not to be the case. Even within one school teachers and TAs can vary so much that a child's experiences from one year to the next can be totally different, sometimes stimulating, sometimes rather unsettling.
In a good school the transition R to Yr1 should, at the start of the new term, be seamless and a child should sense little change. By the end of R they should have learnt to sit still, listen to instructions, started to contribute to discussions, etc, and if they can do all that then Yr1 should pose few problems. Yes, there will be more 'formal' lessons, more subjects with science, history, geography, ICT being introduced. A child will also be expected to accept some responsibility for looking after resources, equipment etc, tidying up at the end of a lesson, knowing 'where they should be and when' for meal times, PE, music, etc. But hopefully this should all be phased in gradually, and there SHOULD be some form of Parents' Meeting to explain to parents what is expected in Yr1 and how it all works.
Strong leadership and effective communication is essential to create a good and happy school (as in any other workplace, of course). I compare it to good and bad shops: There is M&S, and there is Lidls - both are doing similar things, but the 'shopping experience' is somewhat different.
Try and keep good relationships with the Head, class teachers, TAs, MTAs, the cleaners and the caretaker (who often knows more about what goes on than almost anyone else!).
MN parents tend to 'fret' much more than they need, in my experience. Just relax and enjoy sharing with your child, the new class and all it has to offer!
SORRY: I haven't rambled on like this for a long time!