So if I'm understanding this correctly, this is sort of an extreme version of block scheduling, or something like medical school modules. Some university classes are also run this way, usually late afternoon/evening classes aimed at students with daytime jobs, compressed courses during mini-terms, or high school students making up credits in night school. (Some colleges around here offer two-week, daily, four-hour, three-credit courses during the winter interterm.) I can understand why they might choose this structure, as it is about as close to true cohorting as one can get at the high school level, but I'd agree that some students will be challenged to focus on, let alone absorb, instruction in three hour blocks, even with mask breaks (I would assume it's not really a six-hour class, since presumably they get to eat lunch).
I will also offer that this is more-or-less how we homeschooled one of ours a bit ago. In your case, it will also undoubtedly be easier to do in person than online.
It's not a model I would dismiss out of hand, since it has a reasonably functional track record in certain populations, but where those are usually self-selected populations, it's difficult to project how it will play out across an entire high school.
