Chronicles of Nick by Sherrilyn Kenyon
Teenager Nick Gautier thinks he knows everything. Streetwise, tough and savvy, his quick sarcasm is renowned. But his whole world is suddenly turned upside down on the night his best friends try to kill him.
Saved by a mysterious warrior, Nick is sucked into the realm of the Dark-Hunters - immortal vampire-slayers who risk everything to save humanity - and he quickly learns that the human world is only a veil for a much larger and more dangerous one that's filled with all kinds of evil. However, before he can even learn the rules of this new world, his fellow students start turning into flesh-eating members of the undead.