Callistemon
If a teacher needs to shield for medical reasons, fair enough.
It's not just teachers and other staff needing to shield, who are going to have to fight for the right to shield anyway.
It's also about all the contacts.
Think about the implications of just one infected teenager in an average comprehensive school. They will not be able to socially distance (despite what Gavin Williamson claims). They will be couped up in conditions which are ideal for spread.
One infected pupil could be asymptomatic for a couple of days and during that time infect those sitting nearby - maybe three or four. They will all be asymptomatic for a few days, so will take the infection home and possibly infect family members, some of whom are going out to work, who them pass it on, ad infinitum.
By the end of the week, there could be a few dozen infected people - all because social distancing wasn't possible in school. The school will need to close anyway, so the aim of getting the economy going will be a waste of time.
There needs to be efficient and quick testing on a regular basis and provision for one week on/one week off, so that social distancing can take place.