Absolutely reasonable. Older children can and do transmit the virus similarly to adults (the only saving grace with this virus is that children don't often appear to be badly affected individually, but they're still bodies that pass this around the community to those who are likely to become more badly affected) and will be in enclosed spaces for long periods of time in large groups.
DfE says other measures are in place, but they aren't enough and no one thing will eliminate this, so it stands to reason to try everything.
Segregation across year groups, or 'bubbles', helps with knowing contacts but don't offer protection from catching the virus in the first place and thereby passing to families and the wider community before tests take place.
Enhanced cleaning of the environment and handwashing can only help so much; it's sensible to do as many things as possible, to mitigate the risk.
Other workers (not expecting to deal face to face with positive cases) have been given the benefit of all those using their services having to attempt prevention of transmission. Just because education is essential, teachers shouldn't have to teach in risky conditions. The job of a union is to protect people at work, and the government guidelines currently allow only some adults in this county to work in 'covid-secure' workplaces, and do not help to protect others.