Could this be the one you're looking for?
The Rose by Coalitiongirl
The Watchers Academy- where young potential slayers and watchers go to train in demonology, magic, and combat so that they can someday fight vampires, demons, and the forces of evil. There are evil magic professors, a principal with a secret, and an unbreachable wall between the slayers and the watchers. And in the basement, imprisoned vampires scheme and dream of destroying them all...
http://www.dark-solace.org/elysian/viewstory.php?sid=3006&warning=NC-17