Yurikuma Arashi

Far off in space, the asteroid Kumaria blew up, sending bits and pieces to rain down on Earth. For reasons unknown, this causes all the bears ("kuma" in Japanese) to simultaneously rise up and begin attacking humans. Finally, a giant "Wall of Severance" was built to divide them from each other and end their ceaseless war and hatred. In the human world, at Wuthering Heights Academy, Kureha and Sumika are friends who've fallen in love. When they find a lily blooming in their favorite flower bed, their eyes meet...and all at once the Bear Alarm starts ringing, signalling that the human world is under attack.

Synopsis from Anime News Network