Nineteen-year-old McKenzie Parrack was surprised when she took a human sexuality course at George Washington University last year as a freshman.

Parrack learned a lot more than she expected, she said, and she believes the knowledge will help her make informed decisions. And, she said, she realized that many classmates entered the course knowing far less than she did.

Her conclusion: When it comes to sex education, "some people are just living under a rock."

Sex education is hardly a new subject in the United States, said Jeffrey P. Moran, whose 2000 book, "Teaching Sex," chronicles its history. In the 1800s, sex education ran along the lines of warnings that lust could be stirred if people ate highly spiced foods or too much meat or, of course, drank alcohol.

The subject developed in schools in fits and starts, and although the details have changed, remarkably similar patterns can be seen. Look back to 1912, and this is what was happening: With sexually transmitted diseases rampant and nothing but arguments abounding on how to teach young people about sex, a group of prominent educators -- including former Harvard University President Charles W. Eliot -- assembled at a conference in Washington to see if they could find the right approach to settle the matter.

Flash-forward nearly a century and it is clear that they didn't. Newspapers are filled with articles about public school systems' battles over K-12 sex education, even in so-called politically liberal areas including Montgomery County, whose school system is revising its sex-ed curriculum because its old program -- which included such materials as a videotape that uses a cucumber to instruct 10th-graders on condom use -- was challenged as inappropriate.

The issue of what is appropriate has also rattled some colleges and universities, where sex-education classes use far more graphic materials than do high schools and take students through doors their parents never entered when they went to school.

"Over the last few years, there has been an increasing attack on sexuality education in the college classroom," said Gilbert Herdt, director of the National Sexuality Resource Center, based at San Francisco State University, and director of the Program in Human Sexuality Studies. "The result is that college professors are more careful about what they teach," he said.

At George Washington University, a debate among students that was sparked by this summer's dismissal of a popular adjunct professor highlights part of the national discussion.

Michael Schaffer had been teaching human sexuality for 17 years until this past summer, when, he said, he received an e-mail from an administrator telling him that he was being let go.

Schaffer said he is not sure why he lost his job. He was told to look at his student evaluations, but he said they were mostly positive. One unidentified female student, however, blasted his class, he said, saying he "shows naked pictures and videos" and assigned papers that were only an excuse to delve into students' personal lives. Schaffer intends to meet with administrators to learn the full reason for his dismissal, he said.

Patricia Sullivan, acting chairman of the exercise science department and the administrator Schaffer said notified him of his dismissal, did not return phone calls.

Schaffer said he did show such material and talked about sexually transmitted diseases and any sexual subject a student wanted to discuss.

But, he said, there are different ways to interpret the material. For example, he used a videotape that was described in the student newspaper, the GW Hatchet, as "a video of a male and female masturbating to orgasm." Schaffer said that the video was intended to show the physical response to orgasm and showed two people in a laboratory, with sensors attached.

He also said he assigned papers with personal topics because students learn best when material relates to their lives. Dozens of former students who agreed with him complained to the school about his dismissal, including McKenzie Parrack, who said: "I think we are mature students who should be exposed to new and different things. It's not like we are in high school anymore. The classroom is the best place to be exposed to sexually explicit things because it is in the right context."

But sophomore Kevan Duve, a columnist for the Hatchet, saw a bigger issue. Although Duve opposed Schaffer's treatment by the university, he questioned the content of the course, saying that there wasn't enough substance and that it was more "group therapy" than academic work.

After researching the curriculum, he said, "My conclusion was that the class was entirely too frivolous to be part of any university's curriculum."

Duve said that students should have learned about sexually transmitted diseases in middle or high school and that "the university can't be expected to fulfill that very basic function." Instead, he said, students should learn from the readings of major thinkers on the issue, from Plato to Kinsey.

"We have to look at what a university class should be," he said.

Parrack disagrees. She said she took sex education in high school in Texas, where public schools are required to focus on abstinence, a form of sex education that has been growing for a decade as the country becomes more politically conservative.

She said she remembers spending less than one day on learning about sexually transmitted diseases. "Why shouldn't universities teach it?" she asked.

Indeed, what they should teach became a statewide issue in Kansas two years ago after a student, who worked for state Sen. Susan Wagle, complained about a veteran University of Kansas professor's sex-education class. Wagle unsuccessfully tried to deny funding to the professor's department, but she succeeded in persuading her fellow senators to adopt a policy that forced the school to publicly state what was being discussed in class. The University of Kansas cited academic freedom and continued what it was doing.

Various posters warn of risky behavior. In the 1800s, sex-education teachers linked lust to consumption of spiced foods, too much meat or alcohol.