The proper answer is that once you have sexual intercourse, you will keep exposing yourself to the risk of STIs and pregnancy.
Abstinence is a good idea for teenagers because it prevents unwanted teen pregnancies and prevents contracting and spreading STI's. However they could just practice safe sex which should be taught more in schools than abstinence. Teenagers are exploding with hormones and they want to experiment and enjoy themselves and figure out who they are, forcing abstinence will just lead to anger and resentment and possible sexual aggression in adulthood. Abstinence will prevent pregnancy and the risk of STI's but so will proper sex education.