If everyone who is in (in a class where 75% are in) are vulnerable that means a really large number of kids in this country are vulnerable.
We need honesty here really - either the government has made the definition so wide that people who don't really need to have kids in are in are anyway and it's a way of opening schools whilst claiming they're shut, with the issues around transmission that will bring (and still no funding for school safety as far as I know). Or we have a very serious problem with vulnerability and mental health of children - and this needs URGENT action and funding, and not a reliance on schools to plug all the gaps of underfunding of other services. Because this will never work - teachers are overstretched as it is.
The situation as it is in those schools with more than a minority in risks making the children that aren't in vulnerable because it's seriously damaging to those children to be in the minority, isolated, alone and knowing it. It's one thing to feel like you're 'in this together' it's another to be a child struggling at home and not understanding why you are in a minority really struggling whilst everyone else gets to go to school. That has really unpleasant connotations of segregation and children aren't idiots, they question of course if most children are in not them. I think it could lead to really serious mental health problems - feelings that they are 'not worthy' when everyone else is. Feelings of exclusion. Depression, anxiety and serious lifelong mental health problems.
And not forgetting that SOME vulnerable children don't have parents who have the bandwith to complain to the school, to get the LA involved. If that child isn't falling too far behind, isn't complaining, is quiet, there will be seriously vulnerable children that will fall through the cracks. Even more so if they're at home and overworked teachers are not finding the time to check on them. They simply wouldn't know if there was a problem, how would they?
Rotas for all would be much fairer, and safer for those vulnerable children.