The issue is liability.
Headteachers are being asked to follow very vague guidelines and make huge discretionary calls about the health and safety of staff, children and parents.
The layout and facilities of school are all so different so suggestions that you can do things this way or that way are often impossible for schools to comply with.
The government hasn't taken any of the responsibility of this.
If an incident were to happen it leaves schools completely exposed and they may not be properly insured as a result. The schools can not reopen fully if they don't comply with insurance policies and public liability requirements.
For example there is talk of screens in corridors to separate them. So what if the screens fall or if there is a fire?
The threshold of risk is different to that of a hospital for example because you have so many minors to account for.
You also have issues with bottlenecks as entry points to school grounds - child safety means you have few entrance points to grounds for drop off collection so staggered drop off and collection may not be helpful anyway if you are unable to set up a one way system into and out of the grounds.
And there is provision of toilet facilities if you have children working outside and not allowed into certain spaces inside because they are being used by another group.
And there is the SEN issue where teaching assistants are there to manage kids with behavioural issues on a one yo one basis. If they are redeployed as teachers (which they are not trained for nor paid for), does that put staff and other children at risk (given one of the incidents I know has happened in my sons class this year, this is a particular concern of mine).
And that's before you consider the disruption and whether it will produce anxiety. The last week of school before lockdown was horrendous and the kids were really struggling with it. Even the youngest ones.
Whilst there are risks to children not being in school, whilst a child isn't in their care they are not the liability of the school. They have a duty of care to do what they can but their are limits to that. Once they are on the premises though they have a far higher level of responsibility which they may genuinely be unable to fulfill to a satisfactory level.
It leaves some in the dreadful position of knowing that some children are at risk if the school is closed but the risks of opening the school mean they can not do so under the circumstances being suggested.
I think it's poorly thought out by government who really have no idea about how school operate on the ground. They really are regarded as a baby sitting service first and foremost and the total lack of respect for teachers (and children) is a sight to behold.