Just my musings, but I can’t think where the legal obligation to sit through online classes would come from. You have a legal obligation to ensure that your children are educated. Usually you delegate that to a school. You then commit a crime if the children are on the register but don’t attend ‘regularly’ which means ‘on every occasion that the school requires it’ (see the Jon Platt litigation). Obviously if the school authorise an absence for illness or whatever then no crime is committed.
But, the pandemic has changed everything. The government has made it impossible for your children to physically go to school! The question is, as you are still on the register, is your legal obligation to ‘attend regularly’ (a) extinguished by the fact that you cannot physically attend, with the right to decide what they do returning to you, or (b) transformed from physical attendance to whatever activity the school specifies, here, virtual attendance?
I think the answer is clearly (a) and that if you can’t physically attend school, then the legal obligation to ensure they are educated rests on you, but you don’t have to do anything specific. However, some schools and the insane and incompetent education secretary might perhaps disagree with me and feel you are legally required to do anything they feel like telling you to do 🤷♀️ no matter how changeable and unhelpful.
Anyway I certainly would not be doing online schooling of the type you describe, it sounds actively harmful. In your shoes I would write to the head saying that you have tried it but it does not work for your children and no genuine education is taking place, therefore you will be educating them offline and are happy to send weekly reports. If the head doesn’t like it you can just deregister now (a one line email to head will suffice) and reregister when schools open (application form).