Manga got cancelled early so there is no material to make it 24 eps. They actually used almost everything from manga they could.
My only regret is that they totally omitted 31st chapter, where we see how a relationship between Arihara and Yamamoto turned out. I recommend reading it for everyone who watched anime. Thats a real shame that they didnt animated it.
kimikiss gets boring real fast. this series is better because the characters are more interesting. i want more, but idk how i'd feel about a 2nd season. i wish it were 24 eps instead of 12. every thing felt very compressed =/