candlelight2:
First of all - as others have said - banding into ability groups and mixing years is not uncommon. Although I would suggest that initially in Class R, your DD is likely to work only within this age group.
I think you are presuming that every incremental improvement is monitored - and the reality is that monitoring progress (at least at our school) is much less refined than that. I also think that allowing bright Y1 pupils and struggling Y2 pupils to work freely within their ability level and not be the one racing ahead/ lagging behind is easier on both the school and the child.
I'm only a parent, but from what I can work out the TA role is basically to carry out work assigned by a teacher with a particular group (one which may require more support/ guidance or one which due to personalities needs a member of staff to keep them on task). The teacher will most likely move about from group to group - observing general group work and individuals. The teacher (and remember there are 30 pupils) will also have to work closely with individual children to observe achievement of targets/ tasks. [I rather suspect when you visited the teacher was not teaching but talking to your group - probably normally they would be moving about, working with children, recording progress, etc...].
At our school upper ability groups usually work independently with minimal teacher/ TA oversight or input. Maybe once a week and TA/ Teacher will sit through literacy/ numeracy hour with them and really monitor how they're working & understanding. These upper ability groups are also very clear that they can ask for help when they don't understand an assignment or there's a problem - and they do.
So the reality is that TAs (which will have learning support training - but many TAs are actually fully qualified teachers) are just that extra pair of hands to move things along.
If the school is boxing clever they will know that some TAs are particularly good at aiding children at certain stages. For instance teaching phonemes (letter sounds - maybe they're very funny with the hand motions/ songs, maybe they are very patient, etc....) and others may be better at teaching blending (joining different phonemes - Kuh - ah - tuh = CAT) & sounding out words. You may find the TA moves groups as one group masters whatever hurdle and then goes to the next group (of lower ability) to help them master whatever is next to learn for them.
Our school also uses TAs and members of senior management group to support struggling students. They may work 1:1 with them - helping them read, do maths, etc... This is often done in quieter areas of the school - a small teaching room, the library or even quiet corners tucked away in the hallway.
The reality with schools is that it is a collective exercise. I think if you are aware of what notionally a child should be achieving in YR, Y1, etc... then you will be better able to judge whether your child is 'on track' or not; regardless of how communicative the school is or is not.
If you're really concerned - I'd suggest you talk more with the school - but to judge what should be taught & mastered by year in school, I'd recommend looking at Campaign for Real Education's Primary curriculum here: www.cre.org.uk/primary_contents.html. The new national curriculum is still in draft - but you can get an idea from Year 1 - Year 6 here: www.education.gov.uk/schools/teachingandlearning/curriculum/nationalcurriculum2014/b00220600/consultation-national-curriculum-pos/draft-pos-subjects (remember Year R is still under Early Years Foundation Stage curriculum which also applies to under 5s and not the national curriculum).
HTH