Chicago has the third largest public school system in the nation. (Getty Images)

While most U.S. public schools start sex education in the fifth grade, sex education will be coming to Chicago kindergartners within two years as part of an overhaul of the Chicago public schools sexual health program.

The new policy, which the Chicago Board of Education passed Wednesday, mandates that a set amount of time be spent on sex education in every grade, beginning in kindergarten. Chicago has the third-largest public school system in the country, with 431,000 students.

"It is important that we provide students of all ages with accurate and appropriate information so they can make healthy choices in regards to their social interactions, behaviors and relationships," Barbara Byrd-Bennett, the CEO of the Chicago Public School System, said in a statement. "By implementing a new sexual health education policy, we will be helping them to build a foundation of knowledge that can guide them not just in the preadolescent and adolescent years, but throughout their lives."

Under the new policy, the youngest students - the kindergartners - will learn the basics about anatomy, reproduction, healthy relationships and personal safety. Through the third grade, the sex-ed lessons will focus on the family, feelings and appropriate and inappropriate touching. In the fourth grade, students will start learning about puberty, and HIV. Discussions will emphasize that the virus cannot be transmitted through everyday contact such as shaking hands or sharing food.

From the fifth through the 12 th grade, the emphasis will be on reproduction, the transmission and prevention of HIV/AIDS, and other sexually-transmitted diseases, bullying and contraception, including abstinence.

For the first time in Chicago, sex-ed instruction will cover sexual orientation and gender identity. Students will be introduced to terms and definitions associated with sexual identity, including those related to heterosexual and LGBT populations, in an effort to bring awareness, promote tolerance and prevent bullying, said the school board.

Parents or guardians of students can opt out of the sexual health education program if they so choose.

Developed by the Chicago Public Schools Office of Student Health and Wellness last year, the policy was designed to align the Chicago public school system with the standards in President Obama's national HIV/AIDS strategy.