Well, I'm 15 now; maybe I can help? I prefer Romantic, Impressionistic, and Contemporary music to Baroque and Classical; this may affect my opinion. I think that asking the student what kind of music she likes to play is important. This strengthened my own love of piano and music.
Bach isn't a favorite, but I have a friend of the same ages who played an entire English Suite and enjoyed it very much, so it truly depends on the person. I think that learning some fugues also helps strengthen technique and bringing out individual voices. Beethoven Sonatas are fun to play and have different skill levels. op2 no1 seems like a good starting piece for that. Also, I played many Chopin Nocturnes at that age too, as they span a great range of levels and techniques.
When I was 13-14, I played a lot of Impressionistic music. If she enjoys Debussy's Arabesque, which you mentioned (I assume you mean the first and not the second), she may also like Clair de Lune (a well known song, but still beautiful). Debussy also has more upbeat pieces you can look into like Golliwog's Cakewalk. In addition, I played the Reverie after playing the First Arabesque. Faure's Barcarolles are also beautiful pieces.
Many students in that age range seem to enjoy contemporary music. Bartok and Poulenc are two composers by whom I've played some pieces. Copland and Gershwin ("American Composer" style) also have fun pieces.
I don't think the months provide a theme for my pieces specifically, since learning different types of pieces is important in the long run, though it's not bad to concentrate on a specific style at a given time. However, I would suggest seeing what type of piano music she likes in the first place, as it's easier to build from there. If she's not too interested in music yet, it might help to have her play different styles and see what she prefers. =)