Because no one has stated it so far:
The manga is a built-up to the anime. But it kind of works both ways, you can read it after watching the anime or before. I was pleasantly surprised back then to find out there is the manga because I loved the characters and the art to bits.
The anime itself wasn't really good back then and hasn't aged well. It was pretty good with the sinister all-girls school episode, but whatever they tried to achieve with the ending is above my head. And then there was a surprise OVA four years later which wasn't really connected to the rest.