Horitsuba Gakuen

堀 镡 (Horitsuba?), short for 私立 堀 镡 学園 (Shiritsu Horitsuba Gakuen?), Horitsuba Private High School, it's a series of Drama CDs released in the annex of the Shonen Magazine and Young Magazine since 2006.
Before having an Anime version, some mangas have "Drama CDs". In these, they might tell the story of the manga itself or secondary/independent stories. In Tsubasa ~RESERVoir CHRoNiCLE's case, it's different since there is no drama of the manga itself, but of the Horitsuba Gakuen (Horitsuba Private Institute), a sort of alternate universe to both Tsubasa ~RESERVoir CHRoNiCLE~ and xxxHOLiC (hence the name: Horitsuba, formed by parts of the phonetics of the series' names, HORIkku and TSUBAsa), where the characters of both series appear in the quiet and peaceful Horitsuba Private Institute.
Horitsuba, up until the moment, includes four Drama CDs, a little omake appeared in the "Internet Radio Magazine" three omakes in a special number of Shounen Magazine (Magazine SPECIAL). With an original script by Ageha Ohkawa, the CDs were sold only with the Shônen Magazine or the Young Magazine (they cannot be found on stores). The seiyuus are the same (since there already were anime of both series), but the music is totally independent to its animes.
The Horitsuba Gakuen saga narrates an independent story compared to TRC and Holic. Situated in a private campus like the "CLAMP Campus" where there was study centers for all ages (from preschool through college), and even leisure and services. Among the thousand students it's housing, we find Syaoran (an exchange student), Sakura, Watanuki, Domeki, Himawari and both Mokonas.
The teachers are Fye, as a chemistry teacher, Kurogane as a gym teacher, and Yuko as a literature teacher (also the director).