While heading to class last year, Stephanie Cisneros, a Denver-area high school junior, was arguing with a friend about ways that sexually transmitted diseases might be passed along. Ms. Cisneros knew she could resolve the dispute in class — but not by raising her hand. While her biology teacher lectured about fruit flies, Ms. Cisneros hid her phone underneath her lab table and typed a message to ICYC (In Case You’re Curious), a text-chat program run by Planned Parenthood of the Rocky Mountains. Soon, her phone buzzed. “There are some STDs you can get from kissing but they are spread more easily during sex,” the reply read. “You can get a STD from oral sex. You should use a condom whenever you have sex.”

Ms. Cisneros said she liked ICYC for its immediacy and confidentiality. “You can ask a random question about sex and you don’t feel it was stupid,” said Ms. Cisneros, now a senior. “Even if it was, they can’t judge you because they don’t know it’s you. And it’s too gross to ask my parents.”

Sex education is a thorny subject for most school systems; only 13 states specify that the medical components of the programs must be accurate. Shrinking budgets and competing academic subjects have helped push it down as a curriculum priority. In reaction, some health organizations and school districts are developing Web sites and texting services as cost-effective ways to reach adolescents in the one classroom where absenteeism is never a problem: the Internet.

In Chicago, teenagers can subscribe to Sex-Ed Loop, a program endorsed by the district that includes weekly automated texts about contraception, relationships and disease prevention. Through Hookup, California teenagers can text their ZIP codes to a number and receive locations for health clinics.

Many services, like Sexetc.org, a national site run by and for teenagers, offer both privacy and communities where adolescents can learn about sexuality and relationships, particularly on mobile devices, eluding parental scrutiny. Services offer links to blogs,interactive games, moderated forums, and Facebook and Twitter pages.

The messages, rendered in teenspeak, can be funny and blunt: for Real Talk, a technology-driven H.I.V. prevention program run by the AIDS Council of Northeastern New York, teenagers made a YouTube video, shouting a refrain from a rap song, “Sport Dat Raincoat,” during which a girl carrying an umbrella is pelted with condoms.

“When we ask young people what is the No. 1 way they learn about sex, they say, ‘We Google it,’ ” said Deb Levine, executive director of ISIS Inc., an Oakland, Calif.,-based nonprofit organization that administers texting services and checks content for medical accuracy. “But most of the time, the best information is not coming up in those searches.”

Quantifying services is difficult. But Ms. Levine, who hosts Sex::Tech, a conference about sexual health programs for youth, said that requests to make presentations about online or mobile services had soared. Typically, she receives between 40 and 50 applications. This year, she received about 120.

Unlike classroom lessons, which are supposed to follow local, state or federal guidelines, Internet programs have no independent standards. And proponents of abstinence-based sexual education argue that these digital services presume that sexual activity among teenagers is the norm, and do not spend enough time on alternatives.

“They are only focusing on the risk-reduction model,” said Valerie Huber, executive director of the National Abstinence Education Association, which hopes to kick off its online service for teenagers next year.

Those who run digital programs say they simply want teens to have accurate information, to help them make good decisions. Even though popular culture is saturated with sex, facts and advice can be hard to find.

Few disagree about the need for more education. Although the teenage birth rate dropped 9 percent in 2010 from 2009, the United States still has one of the highest rates among developed countries, according to the Centers for Disease Control and Prevention. Rates ofsyphilis, gonorrhea and chlamydia among American teenagers continue to rise.

Most online services receive grants from philanthropies, like the Ford Foundation, and health and education agencies on the state and federal level. Classroom content is largely controlled by school districts, but it is a low priority in many areas. Chicago, for example, does not have a mandated sex education curriculum, although teachers are encouraged to include material in science or physical education classes. School officials see programs like Sex-Ed Loop, which began in September, as vital.

Mary Beth Szydlowski, the H.I.V. education prevention specialist for Chicago schools, said that Sex-Ed Loop not only reinforces what students learn in class but can reach all teenagers, including dropouts. It is managed by the Illinois Caucus for Adolescent Health, which enlists Chicago teenagers to create the text messages as well as blog posts and testimonial videos for its site.

Juan Chavez, 19, a sophomore at DePaul University, remembers sex education during ninth-grade health class as awkward.

“The teacher had been a nutrition major,” Mr. Chavez said. “He was really uncomfortable. He just said, ‘I don’t believe you guys should be having sex, so I’ll just say this because I have to.’ ”

Now, through the Illinois Caucus for Adolescent Health, Mr. Chavez texts and blogs, with a focus on gay teenagers, about such subjects as what to do if a condom breaks, which clinics are gay-friendly and where to find low-cost lubricants — “things people need to know on the fly,” he said.

Parents who fear that sex education will encourage a child to experiment are misguided, said Elizabeth Schroeder, executive director of Answer, a national sex education organization that oversees Sexetc. Studies show the opposite is true, she said.

But making sure that Web-surfing teenagers find these programs, rather than pornographic sites, has been challenging.

Leslie Kantor, vice president for education at Planned Parenthood Federation of America, said it was expanding its chat program, which teenagers can use with handheld devices or online. The organization is trying, she said, to embed material with search terms used by teenagers.

“How do I write content that says ‘sex’ 80,000 times so our page will pop up in a kid’s search on Google near the top?” she said.

When it comes to marketing, programs are increasingly relying on the customer: teenagers.

Real Talk held a classroom contest to see which student could send the most texts containing this prevention message: “ROFL!!!” (Translation: rolling on the floor laughing). “STDs and HIV can spread as fast as this message. Still laughing? Pass on the message not HIV/STDs. 518-HIV-TEST.” Within an hour, the message had been sent to nearly 450 phones.