They All Had A Chance by Michele Leathers

Jane thinks she understands the game, but she'll soon find out one of her opponents is a killer playing by a different set of rules.  

Plain Jane Flowers is a seventeen-year-old summer camp counselor. This is her second year at Camp Sunshine, only this time there won't be any campers. Jane will be competing with other counselors in a series of challenges that will test her physical strength, endurance, and mental acuity. If Jane wins, her entire life will change. People will finally admire and respect her, and nobody will call her Plain Jane anymore. Jane teams up with some of her fiercest competitors, but soon mistrust causes a fracture within her alliance. Someone is sabotaging the games. The stakes have never been higher, yet quitting isn’t an option. Jane will have to play the game in order to survive.