In Japan, the marring age for girls is 15-16, while the boys are 18-19, and CLAMP has had another teacherxstudent in there other works, more so in CCS, TouyaxKaho, EriolXKaho, Sakura's mother and father were even a student teacher relationship. The manga Suki also has that going on, along with Chobits, so Rika and Terada relationship isn't that strange in Japan, considering a teacher can be 20 years old in Japan. Here in America, Canada or anyother place in the world, this may seem disterbing, but it happenes. CLAMP likes to put real world things in their manga, even if it might cause an uproar over what it is over seas, but this is something we should understand, that it is in Japan and a Manga/Anime.
I hope this clears things Just a little bit.