Just a thought, please don't flame me..
So I was thinking, perhaps secondary schools could hire large marquees of the sort used for weddings, fairs etc. to use as temporary teaching spaces. These would be set up on playing fields with the sides left off for fresh air. Pupils could be instructed to wrap up warm. With individual desks spaced out well surely this would be a lot healthier than a bunch of teenagers all squashed into some sweaty classroom.
Obviously this could only work for subjects not dependent on technology, perhaps English or Maths. Teachers would rotate between tents with pupils staying in situ.