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.”