I don't think that astronomy is either a theory or a fact. It's a field of study.
Whether you approve of the stickers or not is not the issue. The decision to affix the stickers belongs to the school board, not to you. The question before the Court is whether a sticker that reads "This textbook contains material on evolution. Evolution is a theory, not a fact, regarding the origin of living things. This material should be approached with an open mind, studied carefully and critically considered." constitutes an unconstitutional "establishment of religion." It obviously does not, and I am frankly flabbergasted that any reasonable person would consider that it does.
I don't think that astronomy is either a theory or a fact. It's a field of study.
Evolution is a field of study also. I took several classes in it in graduate school and studied it for my Ph.D. exams. A lot of people work in the field as well.
I think you are arguing from a religious, not a scientific, viewpoint.