The advice is that where possible social distancing should be adhered to. Not that it should be abandoned altogether. Year 6 should be adhering to this as much as possible. Year 1 and eyes need more space without tables to allow them to move and play without being on top of one another.
The 15 was suggested as a maximum not something the bubbles must be.
We are aiming for 10 per classroom, as our rooms are not particularly big and that's how many we can fit in and have the tables set out to allow 1 per desk with the children approx 2m apart, but still leave room for the teachers to move around and the children to move around and access the limited toys etc.
We are having two adults per class - where possible a teacher and an LSA, though the combination may vary. With the exception of two job share teachers they are the same staff in the bubble all day every day.
Two staff members means the classes can be looked after better. It allows the adults to go to the toilet and have a break for lunch. It means one can be doing the additional cleaning after lunch or toilet visits too. It allows some children to go outside and some stay in - especially useful for bubbles where there are different age groups, such as Kw groups. It also means that the teachers can take a little time to monitor online remote learning from others in their class, or to do some planning.
If you only have one teacher per class then the teacher can't leave to go to the toilet even. It's being practical.