Within YA, proper horror is a hard thing to find.
Scared to Death (Hell's Underground, Bk 1) by Alan Gibbons is really good. I haven't read book 2, but book 1 was a lovely dark tale with Ripperesque elements, demons, murders, & tension.
The Devouring was a nice read, and it's on the decidedly creepy side.
My 15 yr old daughter tells me Darren Shan's Demonata series does the dark beautifully too. (I haven't read it, but she usu has good taste.)
I don't know that I'd call Forest of Hands & Teeth horror, but it's dark & fabulous & well worth the read regardless of whatever genre classification one assigns there.
Some of the classic horror--the masterful S King--is very accessible to YA readers. The teens I know that want to read horror default to him.
And, although it's NOT YA I have to add a rec for the wonderful creepy nasty brilliantly crafted Heart Shaped Box (Joe Hill). His might be the best horror I've read in years. . . actually it's just one of the best books I've read in years.
Good luck. If you find any other great YA horror, please do post them if you get a chance. I'm always on the lookout for the deliciously or disturbingly dark & have had a lousy time finding much true horror in YA.