Well the first thing I would cover would be the negative consequences of choosing to engage in sexual contact. The diseases you can get or surprises like kids would affect you for the rest of your life. The benefits of being abstinent is that you can enjoy this later with "the one." The steps you can take in avoiding a sexual relationship is not falling into peer pressure and just saying no. Even if needed the best thing to do would be to end the relationship. High school is a time for discovering yourself and finding out who you are. And more importantly focusing on laying out a stable future for yourself. (future job/career)
Good Luck! :)