New York City high school and middle school students will be required to take sex education classes starting this fall, the schools chancellor announced last week, the first time the city has broached such a mandate in over two decades.

Until now, schools were required by the state to educate students about HIV/AIDS. Whether or not to include lessons on broader sexual health topics in the curriculum — like sexually transmitted infections, birth control, how to properly use a condom — was left to the discretion of each individual school. The current requirements, Chancellor Dennis Walcott wrote in a letter to principals, is “leaving us with an uneven system that I believe does not serve our students well.”

The new school curriculum requires one semester of sex education in middle schools, and another in high school. Principals can decide in which grade the semesters will be taught.

“We have students who are having sex before the age of 13; students who have had multiple sexual partners; and students who aren’t protecting themselves against sexually transmitted diseases and HIV/AIDS,” Walcott wrote.  “As a parent and a grandparent—and as the person responsible for ensuring that all of our public school students receive a high-quality education—that is very concerning.”

The change is stemming, he said, from a larger, multimillion dollar program launched by the mayor earlier this month called the Young Men’s Initiative, which aims to eliminate the social and economic disparities faced by black and Latino youth.

According to Health Department data, just over 25 percent of teenage girls, and 10 percent of boys, who were surveyed by the South Bronx District Public Health Office, said they did not use a condom the last time they had sex. The Bronx had a higher teen pregnancy rate in 2009 than any other borough, another report found—105 of every 1,000 girls.

Sex education in schools can mean the difference between a young woman graduating or not, said Nancy Biberman, president of Bronx nonprofit WHEDco, which runs an afterschool program for teenage girls called Just Ask Me, or JAM, based around advocacy for sexual health education in schools.

“It’s just another tool in the tool kit of helping kids succeed,” Biberman said, adding that the city’s lack of a sex education mandate for the last 20 years has “significantly hurt a couple of generations.”

JAM started in 2007, when a group of middle school girls at one of the WHEDco’s afterschool program started discussing just how little they knew about sex.

“We came to realize that there was a huge problem in the schools,” Biberman said. “The young people were saying, ‘There’s a lot we don’t know. The information we’re gettting about sex is coming from music videos and things we’re hearing on the street.’”

That same group of girls started a petition and lobbied their principal, at Bronx’s IS 218, to include sexual education in their health lessons. They were later asked to testify before the City Council when the idea of a mandate was being considered by the DOE.

Not everyone, of course, approves of the change. Bronx Sen. Ruben Diaz, Sr. accused Mayor Bloomberg of intending to “completely usurp the role of parents,” in teaching their children about sex.

Parents have the right to opt their children out of classes on pregnancy prevention and birth control, Walcott said.