Last time, the school - in conjunction with local public health - decided on our maximum safe capacity, based on space. It was about 35-40% of our normal capacity.
80-90% of our pupils were eligible for keyworker status, due to the peculiarities of local employers. However, the vast majority of those keyworker parents were working at home, and many only had a single keyworker parent with another at home.
We prioritised rigorously - we pro-actively offered places to all EHCP children, all who were known or suspected to be vulnerable, and all who didn't engage with home learning in the first lockdown ( luckily, we had kept detailed records of every child's work). We then asked all parents to submit evidence of keyworker status and whether they wfh or not, and prioritised, offering what we could up to our maximum capacity.
We also worked extremely hard on equity for both groups - all teachers taught planned and taught remotely, children in school just had that presented on the class IWB. Children in school were supervised by support staff (ranging from TAs to lunchtime supervisors) but were not taught. All marking of work was done by the teachers remotely, as all children's work was uploaded to the same system, and teachers followed up through small group catch-up via Teams. Children in school were kept socially distanced and bubbled indoors, with no access to communal areas of the school. Outdoors, yes, they could play (and eat) within their bubbles, while children at home had a social meet on Teams with the class teacher every day, which the in-school children didn't join. There was a real push to get equipment out to children at home - things like art equipment etc was put out in bags for families to collect while walking past.
As a result, quite a few families initially thinking 'school would be best' ended up keeping children at home as it was clear they were getting nothing 'extra' from being in school.
If all schools have huge numbers of people wanting keyworker places, then it is very likely that public health will step in again and there will have to be a priority system.