I am a senior in high school and have a free credit that I'd like to fill with an independent study next semester. It will be some kind of music class, but I haven't decided WHAT. I've thought about orchestration or music history.

I'm looking for ideas on subjects I could base the independent study on and texts that I could use to supplement them. Help would be appreciated.

Those are really good topics that you have in mind. :) I would recommend using college textbooks for studying these topics, as they will help you in the end.

I would also suggest the following: Music Theory (I'm not sure if you've done Music Theory III material, yet; if you haven't, I would highly suggest doing this material before you study anything else). I'm teaching myself Music Theory III material, and it's an adventure, to say the least. I'm using the textbook titled Tonal Harmony with an Introduction to Twentieth-Century Music, 6th Edition, by Kostka and Payne, and it's extremely in-depth (listening CDs and everything). The analyses in this class are extremely in-depth as soon as you learn new topics (Neapolitan Chords, Augmented Sixth Chords, Mode Mixture, Chromaticism, and Chromatic Modulation (I find the modulation techniques learned in this class extremely interesting)).

If you have already studied Music Theory III material, I'd go with Music History - the university that I attend uses A History of Western Music by Grout, 7th Edition and the Norton Anthology of Western Music, Volume 1, 5th Edition.

For Orchestration, I would recommend Samuel Adler's text.