I don’t think the George and Charlotte are self isolating rather the pupils that have shown symptoms and are being tested are self isolating.
That's how I read it as well. The 4 pupils are self isolating. There is no reason for George and Charlotte to. If they were exposed tahen I would expect the entire Cambridge clan to self isolate for the next few weeks.
I imagine anyone recently traveling with a fever is being tested. As it was just break week I'm sure many families traveled. There's no reason to jump to conclusion that anyone has that virus strand at the moment is at the school until a positive test comes back. Cold, flu, strep, etc are all likely as well.