I had another thought or two...
Would something like the MIT open courseware site be useful to you? There are quite detailed syllabi there of various courses (including astronomy), recommended texts, reading lists, sample exams, and so on.
There are also things like the IGCSEs and Cambridge PreU programmes that have syllabi and suggested texts--astronomy seems to be nestled inside of the physics courses there, but you might glean a few ideas about scope and sequence as well as locating some useful resources.
Would an online university be of interest? (I am assuming there are such things in the US--lots of Canadians use Athabasca University, for instance, for online post-secondary studies--they officially accept students as young as 16, but there is a mechanism in place to request a special admission for a younger student. Athabasca has four astronomy offerings; I would think something similar must exist south of the line, too.)
