Ideally, an adolescent should recieve sexual health information from their parents, from education, and from healthcare. As professionals, we should make sure we are delivering the information young people need to be informed, prepared, and healthy.
According to Doug Kirby of the National Campaign to Prevent Teen Pregnancy, effective sex education should have the following characteristics (see the source):

• Focus on reducing one or more sexual behaviors that lead to unintended pregnancy or sexually transmitted infections, including HIV.
• Deliver and consistently reinforce a clear message about abstaining from sexual activity and/or using  contraception. This appears to be one of the more important characteristics distinguishing effective from ineffective programs.
• Provide basic, accurate information about the risks of teen sexual activity and about ways to avoid intercourse or to use methods of protection against pregnancy and sexually transmitted infections.
• Include activities that address social pressures that influence sexual behavior.
• Provide examples of and practice with communication, negotiation, and refusal skills.
• Incorporate behavioral goals, teaching methods, and materials that are appropriate to the age, sexual experience, and culture of the students.
• Employ teaching methods designed to involve participants and have participants personalize the information.
• Are based on theoretical approaches that have been demonstrated to influence other health-related behaviors and identify specific important sexual antecedents to be targeted.
• Select teachers or peer leaders who believe in the program and then provide them with adequate training.
• Last a sufficient length of time (i.e., more than a few hours).