Weekend by Christopher Pike

The first incident happened the year before when Robin Carlton ingested a large amount of insecticide, permanently damaging her kidneys and holing her up in the Carlton Castle. No one has forgotten, but all have moved on. Eventually, in a viscous cycle, Robin's poisoning comes back around on the eight teenagers that were at the party that night last year after they're invited to Mexico to stay at the mansion. What they think is going to be the perfect weekend ends up turning out to be horrific. Someone wants revenge for the girl that's fighting for her life upstairs and the terror won't stop until the weekend ends.