It started when Alice Dreger’s ninth-grade son came home from school and told her that his sex education class was going to be focusing on abstinence.

Dreger, a writer and professor of medical humanities and bioethics at Northwestern University, has always been frank with her son about sex (he apparently knew what menstruation was in preschool). She opposes “abstinence-only” sex education, and her son was skeptical, too, according to an essay she wrote for the Seattle alt-weekly “The Stranger.” Together they found a Web page that summarized research on the issue for her son to take to school the next day. Curious about what he would be taught, Dreger went with him.

The policy in the school district in East Lansing, Mich., allows parents to attend such classes, so Dreger took a seat in the corner and brought out her laptop. And then she began to tweet.

When her son brought up the research he’d printed out, a visiting sex ed presenter dismissed it, Dreger wrote. “You can look up anything on the Internet,” she told him.

The class went on.

Dreger continued to tweet for the rest of the hour-long session (her full account is compiled here), and during her argument with the guest teacher when the class was over. By the time she left the classroom, everyone at the “locker commons” was talking about what had happened, Dreger’s son said. So was much of the country — Dreger’s tweets had gone viral.

Colby Fletcher, the principal at East Lansing High School, criticized Dreger’s behavior in a statement to the Lansing State Journal.

“I support the appropriate expression of a plurality of viewpoints; however, I am very concerned by the utter lack of civility I see conveyed in the tweets and the behavior the tweeter admits to exhibiting in the classroom. This is not representative of the conduct we expect to see adults model for our students.”

He emphasized that the school’s sex-ed program is abstinence-based, not abstinence-only. “Abstinence-based instruction teaches that abstinence is the only way to be completely safe,” Fletcher said in the statement, according to the Lansing State Journal, “but the curriculum also reviews contraception choices. This parent attended on a day where abstinence was being taught.”

Michigan Public Schools policy requires sex-ed programs to “discuss the benefits of abstaining from sex until marriage and the benefits of ceasing sexual activity if the pupil is currently sexually active.” Information on contraception is not mentioned among required components of the curriculum, although the policy states that districts are not prohibited from discussing condoms within their program.

The abstinence class taught at Dreger’s school, called Sexually Mature Aware Responsible Teens (SMART), is provided by an independent contractor for the school, Fletcher said.

“We are trying to give them an option,” Lori Bolan, who administrates the program, told the Lansing State Journal. “We’re just one portion of what the school provides.”

But in her essay, Dreger said that the program was “terror-based” if not necessarily abstinence-only.

“Here’s what these visiting ‘educators’ were telling those kids: Condoms fail. They fail so often, they are pointless. There is no birth control except condoms. So if you have sex, you will end up with a pregnancy, and there is no abortion — you have to have that baby. And you will be shamed,” she wrote.

For his part, Dreger’s son dug up an analysis of several abstinence education studies in the British Medical Journal. According to the essay, he planned to distribute them in class the next day.