In most of the US, fewer than half of US high schools teach all the sex ed topics the CDC recommends, according to an agency report. Only 20 percent of middle schools do. About a third of teens ages 14 and 15 in the US say they’ve had sex at least once — which means that many adolescents are making sexual decisions without the information they deserve.

For high schools, the three best states were New York, New Jersey, and New Hampshire, where more than 75 percent of schools cover all the CDC’s recommended information. The worst state was Arizona, where only one in five high schools taught all recommended topics. And middle schools in general faired worse than high schools did; the CDC couldn't find a single US state where the majority of middle schools met the government's goal for sexual health education.

The worst state was Arizona

Half of all sexually transmitted infections in the US — and nearly a quarter of the HIV diagnoses — occur in people who are younger than 25. And teens in the US are more likely to give birth than in most developed countries; though the teen pregnancy rate is dropping, it’s still relatively high. That’s why the CDC has recommended 16 topics for sexual education; these topics cover how to get and use condoms, how STIs are transmitted, and the health consequences that can arise from HIV and pregnancy. Studies have shown that sex ed doesn’t increase sexual activity. Rather, it often delays teenagers' first sexual experiences and reduces sexual risk-taking.

"Young people are sexually active and that puts them at risk, especially if they don't have the information they need and skills they need to navigate those relationships well — and with good health," says Stephanie Zaza, director of the CDC's Division of Adolescent and School Health. "We have a really big opportunity, I think, for parents and communities to do a better job with providing our young people with the information they need to be healthy."

Some states "have pretty restricted rules about what can be taught."

There are plenty of reasons for the gap between the CDC’s recommended sex ed and reality, Zaza says. Some schools don’t have enough money or qualified teachers for the classes. But sexual education courses are also stigmatized, with some lawmakers advocating abstinence-only programs for sexual health. "There are states that have pretty restricted rules about what can be taught," Zaza says. The schools are responding to social pressures.

For school administrators that find themselves in a jam, Zaza thinks giving people the numbers can work. "I think one of the most important things we can do is to provide the data and make it clear that this is an actual health challenge," she says. In some cases, telling people how STIs and pregnancy affect their community can make parents more willing to provide their children with better information.

"Parents need to be engaged in these issues, not only in helping their children but also in working with the schools to make sure they're meeting their children's health needs," Zaza says. "If we wait until kids are already past puberty, already past the point of sexual initiation, before we start teaching them, we're really too late."