Kyoushirou to Towa no Sora OP - Cross Heart
While the series isn't all that great, though still one of my greatest guilty pleasures, it had some of the most awesome music including its OP and ED. Here I've uploaded the OP performed by the wonderful CooRie for your listening pleasure. It's a very catchy song that I've replayed over and over and over for days on end.
Download/Listen to the song here