Nevermore 

James Jung is the top student at Ozoh Academy, home of the gifted and mega rich. He is also an undercover agent protecting the principal from a dangerous assassin. His name? Samuel Gyo, codename black mamba and now James’ homeroom teacher. In order to fight, they must abide by three rules. First, they cannot have their identities exposed to the other students. Second, they cannot harm the students. Third and most importantly, no explosives or firearms. Poison and tranquilizers are fine. Fighting to the death, no one ever briefed them for the fourth and final rule: you must not fall in love. Will they break it? (Source: Tapas) Note: Includes 10 extra chapter.