Monsters in the Mountains (San Nico Slayers #2) by Donna Taylor:
 
There’s no macrame at this sleepaway camp. Just murder…
 
Jenna never planned on starting her summer vacation by staking a vampire serial killer. She certainly had no intention of joining a supernatural shadow agency. Yet, here she is on her way to Slayer Summer Camp. If the hours-long deep dives into the dark and creepy don’t kill her, the intense physical training probably will. Or maybe it’ll be the ghosts in the woods that really want her to know they’re there.
 
When Jenna’s moods start changing, she thinks it’s just stress. Even Connor feels the pressure, his interactions growing shorter, snippier, and sometimes meaner. But when the ghosts from the woods invade Jenna’s dreams, she knows it’s more than studying overload.
 
As her and Connor’s changes grow darker—and deadlier—she needs to figure out how to get what’s inside them out before they both, and everyone else at camp, end up dead.