A Wisconsin prosecutor is warning sex education teachers they could face charges if they follow a new state law that allows them to instruct students about proper contraceptive use.

A letter sent to five school districts by Juneau County District Attorney Scott Southworth said the instruction could amount to contributing to the delinquency of a minor if teachers know students are sexually active. He said the districts should drop sex education until the law is repealed.

Southworth also argued that teaching contraceptive use encourages sexual behavior among children, which equates to sexual assault because minors can't legally have sex in Wisconsin.

"Depending on the specific facts of a case ... this encouragement and advocacy could lead to criminal charges," Southworth, a Republican, wrote to districts in his county.

The law's chief author, state Rep. Tamara Grigsby, D-Milwaukee, dismissed the March 24 letter as a scare tactic.

"It's beyond ridiculous," Grigsby said Tuesday. "It's irresponsible to portray this act in the way he is."

Southworth said in a Tuesday e-mail to The Associated Press that he "merely provided a legal opinion to my school districts about the impact of the new mandate."

"It was the Legislature that acted irresponsibly," he wrote.

Wisconsin school districts aren't required to teach sex education. But the new law, which took effect March 11, lays out requirements for those that do, including teaching the benefits of abstinence, criminal penalties for having underage sex and the benefits and proper use of contraceptives.

Supporters, including groups representing nurses, health departments and the state teacher's union, maintain the law will help reduce teen pregnancies. Conservative opponents counter schools should focus on abstinence.

Southworth's letter said law would convert sex education classes "into a radical program that sexualizes our children as early as kindergarten. This, in turn, will lead to more child sexual assaults."

Southworth complained that language prohibiting biased instruction makes it impossible to teach that sexual promiscuity is wrong. He also said a clause allowing volunteer health care providers to teach sex education could open the door to Planned Parenthood employees marketing sexually oriented products to students.

Planned Parenthood doesn't go into schools unless a school asks, said Chris Taylor, public policy director for Planned Parenthood of Wisconsin. Taylor said the law is designed to prevent sexually transmitted diseases and teen pregnancies.

"The real issue here is you have a district attorney who says teachers will be prosecuted," she said.

Southworth's letter is "a friendly warning," said Matt Sande, legislative director of Pro-Life Wisconsin, which registered to lobby against the law.

"He's simply doing his duty as district attorney," Sande said.

New Lisbon Superintendent Tom Andres said his district, which was among those that received Southworth's letter, is seeking legal advice about the law. While the school board will make the ultimate decision, Andres said he believes his schools should teach according to the law if parents approve.

"We're in a moral dilemma," Andres said. "We know our kids need correct, right information. We have to know what that is and teach it in such a manner that doesn't promote sexual assault or bullying."