They're already increasing the hours per day - in mandatory study halls and afterschool reading programs. Hour wise, there's not much more room to grow. Good luck convincing parents they're kids need to stay in school beyond 5:30pm.

The "good" news is that this is largely unsustainable as you've got to pay teachers, admin staff, transportation, and provide supplies for all these ambitious activities education reformers keep dreaming up. Schools are strapped for cash as-is. Hell, my old Highschool currently shuts off most building lights in between classroom periods to save on energy costs. This isn't done because the district is trying to be green: it's being done because it's broke.

Ultimately sheer economics will prevent 6-day/10+ hour school days from being implemented nation-wide.