Regarding astrophysics, look at the course catalog and see if there’s any specific class for that subject. If not, which wouldn’t surprise me at all, and you want to conduct independent study or research in the subject, look up the science teachers and see what their background is. Some might have specialized in school or in industry prior to joining the school. One of my daughter’s physics teachers specialized in some very esoteric subjects, as did a biology teacher.
Make sure you ask yourself what astrophysics means to you and what your current capabilities are. If you want a grounding in calculus-based physics and general astronomy (which usually includes cosmology), those classes are likely available at the schools you’re considering (check the catalog!). Theoretical astrophysics would require fairly advanced mathematics capabilities which you may or may not be able to develop during your high school career (tensor calculus, non-Euclidean geometry, etc). Applied astrophysics (orbital motion problems, an understanding of basic black hole behavior, etc) is within reach with multi-variate calculus, differential equations and some programming capabilities. Research is probably the easiest and simply requires you identifying a suitable subject. Lots of ground out there where amateurs can contribute … though it tends to be somewhat tedious like photographing minor bodies in the solar system and running analyses to refine orbits.
I love that you have an area that you’re interested in and, I’m assuming, passionate about. Astrophysics obviously lies at the intersection of physics, astronomy and math. Again, I don’t know your current capabilities, but building your knowledge in those three realms may take your high school career (and beyond).