Kirarin does not exactly have seasons, it has been airing non-stop weekly from the first episode (OA 2006-04-07).
This is the way the opening & ending themes have changed:
Opening Theme:
#1: "Koi☆Kana (恋☆カナ; Is It Love?)" by Kirari Tsukishima starring Koharu Kusumi (eps 01-26)
#2: "Balalaika (バラライカ)" by Kirari Tsukishima starring Koharu Kusumi (eps 27-)
Ending Theme:
#1: "SUGAO-flavor (Genuine Flavor)" by Kirari Tsukishima starring Koharu Kusumi (eps 01-17)
#2: "Ooki na Ai de Motenashite (大きな愛でもてなして)" by ℃-ute (eps 18-26)
#3: "Mizuiro Merodi (水色メロディ; Aqua Melody)" by Kirari Tsukishima starring Koharu Kusumi (eps 27-)
Following the logic of a new Opening Theme every 26 episodes (= 26 weeks = 1/2 year), the next one is due in April 2007. It could be changed earlier, perhaps depending on the story line.
But the big question is how long will Kirarin continue? Because the anime is adapted from the manga, the producers continue to make new episodes until they run out of material. Perhaps a new song will reflect some sort of "finale" in the story line? On the other hand, another 26 or 52 episodes may be possible depending on TV ratings and merchandize sales. One thing I know, Up Front is not complaining about the CD sales.
Kirarin Revolution page on the Anime News Network:
http://www.animenewsnetwork.com/encyclopedia/anime.php?id=6178