Sky at the top-- yeah, it take a lot of convincing on that one. Just right
now I'm working on weather paintings with 2nd grade. I had them paint the
stormy or rainy sky first with shades and tints. They filled the pages. The
next class when this was dry we painted in a landscape or city scape so it
overlaps the sky. I can get a lot of nice results doing this sort of
prescriptive assignment (of course, they tend to look all the same, which is a
drag). But then when they get a chance to do another painting on their own and
I still a bunch of green lines at the bottom and blue lines at the top.
Pre-drawing-- I've decided that predrawing with a kinder size black crayon
works okay sometimes. They can use the crayon lines as a resist that helps
paint stay where they want it. And they don't make tiny details like they will
with pencils. Predrawing with regular blackboard chalk works well for oil
pastel pictures. You're right that pre-drawing with pencil can be a problem.