As @MasterGland says, the independents would put a spanner in this.
I doubt the government would have the will to push this through, especially for boarding schools. (And there are state boarding schools that would also struggle with this).
It goes against the idea that you have choice in education. I’m not saying that is right or wrong, but some parents cite the longer summer holidays that you get in the private sector as a reason for choosing it.
I can see indies opting out and then they can have their pick of the teachers who are not ideologically opposed to private education.
But I agree with PP that this is not the answer. Targeted investment is needed, as is bringing class sizes down, and effective positive behaviour management for disruptive pupils.
You can’t starve education of funding, then decide that problems will be solved by just switching the timetable around.