Writers on Writing: An Author's Guide is a free book on Amazon with some pretty decent advice from Jack Ketchum and several authors from the modern horror genre. Best of all, it's perma-free. There is some decent advice in there and in the subsequent entries into the series of author guides by the same name.
Essential authors in the horror genre? Depends on what your tastes lean toward. Obviously, Stephen King and H.P. Lovecraft. I don't think much explanation is needed for them. Clive Barker excels at dark paranormal and sexual themes. Jack Ketchum writes extreme horror with blood, guts, and cannibalism well. Paul Tremblay is one of the rising stars in the genre. His books focus on teetering at the edge of the paranormal and non-paranormal leaving the reader to their own conclusions. Brian Keene is a pulp writer and does monsters well. Joe R. Lansdale is a genre unto himself. He writes a lot of paranormal dark fiction.
Also, I suggest watching something like "Twilight Zone" which contains elements of science fiction, horror, and fantasy which don't rely on blood and guts, present themselves with themes still relevant today, and episodes are written by some of the masters of the craft.
I do have a couple of questions for you. What level of schooling is this for? High school? College? Also, is this horror writing club approved? Maybe you have the coolest teachers ever but horror typically contains themes and subject matters which often leave people uncomfortable and/or border on the edge of appropriateness. This typically isn't stuff schools like to indulge their students in.