We teach the importance of exercise, a healthy diet, and good oral hygiene in schools. Why wouldn't we also teach good practices for sexual health?
Schools already teach children about the importance of a healthy lifestyle. We teach them on the dangers of alcohol and drugs, nutrition, the importance of exercise, and oral hygiene.
Sexual education is a matter of public health. Students must be informed of the dangers of unprotected sex and the measures they can take to prevent the spread of sexually transmitted diseases. Morality and ethics aside, teaching teens about contraception is a must for promoting a healthy lifestyle.  The US is in the grip of an STD epidemic. 20 million new STD cases emerge in 15-24-year-olds every year, costing around $16 billion annually in healthcare costs and pose a major threat to the individual’s long-term reproductive health. Failure to teach sex education as part of a comprehensive health program in schools leaves them vulnerable to disease and fertility problems in later life. 
Sex education is not simply a matter of public health and attempting to teach it in this way is damaging and dangerous to the sexual development of young adults. An amoral approach that teaches sex education as part of a public health program separates human sexuality from its connection with love and emotional affection. This can lead teens to behave in a more promiscuous way. It can also inhibit their ability to detect sexual abuse and identify grooming behaviour. Themes like love, value and admiration must be a central component of any sex education. These, unlike alcohol, tobacco, nutrition and exercise, are not matters related to public health. They are part of the emotional and moral development of a child and therefore fall under the educational purview of the parent, not the school.
[P1] Schools have a responsibility to teach matters of public health and the importance of a healthy lifestyle. [P2] Sex education is a public health matter. [P3] Therefore, sex education should be taught in schools.
[Rejecting P2] Sex education is not a public health matter. It is a moral and ethical matter. [Rejecting P3] Moral and ethical education is the parents' responsibility. Therefore, sex education is the parents' responsibility.