'Prime Evil' by Diana G. Gallagher

When attractive, youthful Crystal Gregory takes a teaching position at Sunnydale High, she also takes what seems to be an instant dislike to Buffy Summers. Buffy's Slayer sense doesn't detect anything unusual about the teacher. But when you're living on a Hellmouth, nothing is what it seems.

Crystal isn't above playing favorites, and her pets -- including Willow, Michael, and Anya -- are loyal to her in the extreme. This wouldn't bother Buffy...but for the fact that she has found a fresh burn mark on two of the students' necks. Giles immediately recognizes the symbol as the mark of a powerful witch whose existence predates the Salem Witch Trials.

The primal witch will continue to wreak havoc until she successfully gathers a coven of thirteen people who have magickal ability. Once the group is created, she will perform a moonlit ritual to endow herself with infinite power, at which point all members of her clan -- indeed, all human beings -- will be subject to her will. Buffy's going to need her Wicca girl, Willow, to save the world this time -- but can Willow be persuaded by reason before it's too late?

Notes:
*Featuring Buffy, Joyce, Giles, Xander, Anya, Cordelia, Willow, and Oz

*Published by Pocket Books