I think one of the things that bothers me about the argument that we can't rearrange school schedule to avoid planting and harvest seasons is that we're ignoring a lot of other work done by children, particularly girls. Girls, especially those who are part of the first generation in their family to go to school, are often still expected to do a great deal of work at home. It's not necessarily enough to keep them out of school (although it might every so often), but it is plenty of work.
So why doesn't anyone get up in arms about that? The work girls do at home can be as labor-intensive, time-consuming, and difficult as nearly anything anyone does outside. By the same logic used above, school should be in session very early in the morning at least to keep girls from having to do early morning work. But everyone knows that many girls wouldn't be able to go to school if they couldn't still work at home, so we don't mess with that. Seems like there's a bit of a double standard here.
(For the record, I think one of the biggest goals in international development ought to be easing the work burden on women and girls. Too many women and girls are spending a huge amount of time taking care of basic needs- time that could be spent in other, more productive ways, like education or a million other things. What if the average woman in the world suddenly had 4-8 hours a day to spend doing something else besides hauling and heating laundry, washing clothes, cleaning, gathering fuel, and so many other things? What changes might we see in the world?)