Consent from the person you are kissing — or more — is not merely silence or a lack of protest, Shafia Zaloom, a health educator at the Urban School of San Francisco, told the students. They listened raptly, but several did not disguise how puzzled they felt.

“What does that mean — you have to say ‘yes’ every 10 minutes?” asked Aidan Ryan, 16, who sat near the front of the room.

“Pretty much,” Ms. Zaloom answered. “It’s not a timing thing, but whoever initiates things to another level has to ask.”

The “no means no” mantra of a generation ago is being eclipsed by “yes means yes” as more young people all over the country are told that they must have explicit permission from the object of their desire before they engage in any touching, kissing or other sexual activity.