I've run into the same problems with painting (k-6). I think one of the main problems is their experience with the medium, most of them are still at that manipulation stage. You might want to start with an non-objective/abstract to warm them up. I know this is a tired example but Starry Night is a take off I've used. Suggestion of the landscape and a sky where they can experiment. Also, we focused on tints/shades.
And the sky meeting the horizon ( isn't it fun when the kids look at you with wide eyes and finally "get" it) ... I've already used light blue paper, with the vanGoghs we used dark blue and purple. Depends on how much time you want to spend. I like the idea of sponging or watercoloring the sky the class before also.