School leadership and teachers have to follow the guidance to the best of their ability, they also care deeply for the children they are responsible for.
Imagine they opted for 15 kids in each class with no social distancing to enable them to bring back more children than they would be able to by following the guidance to the letter. They then have an outbreak, how do they justify to the parents of the children or themselves, that they didn't follow the guidance and potentially could have prevented the outbreak if they had followed the guidance.
It's about risk assessment, they need to assign a risk score to each risk. If the score is to high, they think of mitigation they can put in, then restore to see if the risk is now at an acceptable level.
The other thing the schools are very aware of is that the concept of bubbles is flawed by family relationships.
If you have children 1, 2 & 3 in Reception
Child 1 has a brother in Yr 1
Child 2 has a sister in yr 6
Child 3 has no siblings
Child 1 starts coughing, now her brothers bubble is compromised. In her brothers bubble is a child with a sister in a different yr 1 bubble and so on and so on.