Campbell decided to boycott the talk, but junior Carly Thaw was there and recorded it on her iPhone.
"I think everyone started off with interest, or at least giving her the benefit of the doubt," Thaw said. "And she just accelerated as she started speaking and got really loud and kind of abrasive."
"Condoms aren't safe," Stenzel says on the recording. "Never been ... Never will …"
"Girls, you're scarred for life," she says at another point.
"There are a number of pregnant girls at my school," Thaw said. "[There's a] greater number of girls who have had sex before. And for her to come up and tell people that if you've had sex before you're married, you're impure, there's nothing you can do about it, you're screwed for life... It was just, like, What are you saying?"
Campbell conceded that abstinence is the best way to avoid getting pregnant or an STD, and that those on birth control were less likely to use a condom. What she mainly objects to is the tone of shame she heard in Stenzel's presentation.
"Some of the people were crying as they left, because she [was] telling us that we're trash and we're useless if we've ever had sex before," Thaw recalled.
"School is supposed to be a place where you're safe," Campbell said, "not where you should be shamed for what you have or haven't done."
Campbell gave an interview to a local newspaper. The media exposure landed her in Principal George Aulenbacher's office.
"He said, 'How would you feel if I called your college and I told them what bad character you have, and what a backstabber you are?'" Campbell said.
Asked if she felt this was a real threat, Campbell said, "I did."
Aulenbacher has denied that he threatened her.
Campbell didn't stop, and her story appeared on CNN and Seventeen magazine. She filed an injunction against Aulenbacher and the school board seeking to prevent them from possibly retaliating against her, including, she says, by calling Wellesley.
That's when the community turned against her, she said. A Hate on Katelyn Facebook page appeared.
"As I walked down the hallway, everybody went silent," Campbell said. "One of the football players actually spat on me.... I can't think of anything more insulting to do to someone than spit on them."
Was Campbell shocked at the reaction? "I mean, I understood from the beginning that it was a really hot and difficult issue, but I never expected people to outright dislike me for that," she said.
By May, the backlash had become heartbreaking, Campbell said. She and six other seniors were told the graduation program had been streamlined and they wouldn't get to speak.
"It was really a slap in the face," Campbell said.
Neither Aulenbacher nor the superintendent of schools would agree to talk to "20/20."
Kanawha County School Board president Pete Thaw (who is the grandfather of Carly Thaw) said he had no idea who approved Stenzel's appearance. She was paid privately, he said.
The school board passed a new policy on school speakers.
"If you're going to have a public meeting, and if it's going to be about sex, religion or politics, you have to have the O.K. from the superintendent of schools," Thaw said.
Pam Stenzel declined a request for an interview. She continues to deliver her message to teens across the country.
Katelyn Campbell's injunction against the principal was denied, but her scholarship to Wellesley is secure.
Last week she received a national student leadership award. She hopes to become a lawyer for the American Civil Liberties Union.
"I feel like I've been given a unique position to be a vehicle for social change," Campbell said. "I have the mic, so I might as well use it."
Watch the full story on "20/20: Teen Confidential" TONIGHT at 10 ET.