I agree with mrz. It's just bad organisation as far as I'm concerned. I often have my TA working with a group of children, but it is a small group (max 6 children) and they tend to stay in the classroom. The only time she takes them outside is during guided reading because they go out and play noisy games!
The group she takes changes each session (4 sessions per day), according to which group I'm focussing on after the initial whole class teaching focus.
In my opinion, your child's teacher simply has not understood the basic idea about why mixed age classes work - ie. the cross-over of children in the middle. This means that above-average Reception children should be working with the below average (or even, average) Year 1s. Separating them by age is just silly, and extremely lazy teaching as it means that she's only recognising children by age, and not by their developmental stage or understnading of any particular concept.
It will be interesting to see what Ofsted say when they come in.