The school day is long enough already - kids need time to be kids, not tired and stressed by sitting in a classroom all day. I taught EFL in Korea for a while and that is country where the kids start school at 7am and finish at 10pm five days a week - I wouldn't wish that on either of my two.
It's already been alluded to, but assuming that additional teachers were required for such changes, where do they come from? It's a highly trained / low paid profession, so I can't see longer hours attracting more to the profession. On top of that, when are the teachers supposed to plan lessons, mark homework, or do anything else related to their job? It's just not feasible.