@mac12 Heads have some discretion, but not much. I think unless you have a very sympathetic head, they'd be unlikely to authorise this except for a child with pre-existing serious health conditions. If you kept them off for long enough, you would potentially be looking at fines/EWO visits.
I'd think about this in practical terms:
-How long could you viably keep them off for? Does one of you work from home/is a SAHP? It's likely we are just at the very start of this, so you'd be looking at keeping them off until May/June, maybe longer. How realistic is this actually for you?
-Realistically at this stage they are more likely to pick up flu/another nasty winter virus at school- so I'd bear this in mind when visiting your friends. My school had a properly nasty virus doing the rounds last half term, which could have been really bad for someone on chemo. Obviously it might have had a shorter incubation period, but it's still a problem.
-You could consider keeping them off on days that might be especially risky e.g. if there is a school trip to a large event, or an event like a sports tournament where they'd be mixing with lots of other people.
-Speak to the school about the handwashing facilities they have available. Send them to school with hand sanitiser if you can.
Have the school had a ski trip return from Italy, or are you in an area with known cases?