Gah... I'm surprised that nobody's actually commented on this series before!
SaiKano (shortened and commonly used form of the Japanese title) is truly a wonderful story, very well voiced, with a very moving story. Set in a potential modern Japan, at first one would presume this is 'just another' Japanese High School story, about love and relationships. Until the end of the first episode. Of course, that's a major part of it, but this is by no means the same as any other. Within the second half of the first episode, however, we are shown that a lot of violence and action is to be included, of a very moving (and sometimes graphic) nature.
It's amazing how many emotions can be stirred up in just thirteen episodes, but this manages an awful lot. A wonderful series, very recommended. If you don't mind becoming depressed for a week.