Look at the ARM based Chromebooks (OP1, Mediatek). But, you'll be sacrificing performance for battery life. That's an 8 hour day, under normal use that's 6-7 hours. I believe any Chromebook you buy today will get you more than that.
We supply each classroom with a couple of extra chargers, and the students don't normally bring their own charger with them. These are with the Dell 3120 and the Lenovo 500e.