As a former educator, I'm against it.
It's as simple as diminishing marginal returns. It's hard enough to get kids to pay attention and learn in a 6 hour day, and I don't see how an 8 hour day will benefit most kids. I'm sure that for some it will be a advantage, but I don't believe it will be for most. Teachers work alot outside of class, and adding 2 hours to their workday isn't going to improve their productivity. School districts are already underfunded and under supported, and I don't see where they will get the support for 2 additional hours of instruction. And if you are going to take the instruction out of it, then school is just daycare/parenting. We already let the schools "parent" our children enough as it is.
One of the key elements for child development is play time outside of adults. With a 8-5 day, sinner, homework, etc. where does a child have time to play with their friends in a less structured environment? I guess on weekends, but with play being a part of child development, I feel they need more than a couple days a week. Play may seem trvial, but it's where were learn to interact with peers, have accountability to them, take personal responsibility for conflict, learn problem solving, the creation and adherence to rules, calculating risks, etc. There's so much development in something that adults may see as "wasting time". But if all a child's activities are those structured by adults, where do they learn personal accountability, independence, creativity, etc.?