Horitsuba Gakuen is another "What if" from CLAMP, bringing Holic (Horikku) and Tsubasa together into a school situation. Hence the name "Hori-tsuba", or Horitsuba Gakuen.
It's mostly crack. Fai, Yuuko and Kurogane are teachers at the school, and Doumeki, Watanuki, Himawari, Syaoran, Syaoran, Sakura, Mokona and Mokona are students. They've had two misadventures so far in two seperate drama CDs, the first centering around Syaoran (the transfer student)'s confusion about Valentine's Day, and the second around a mysterious doppleganger. The omake was a promotion for the third drama which should be coming out soon.
Hope that was helpful.