School districts in Kansas must get parents' written permission before teaching their children sex education, the state Board of Education decided Wednesday.

The board adopted the policy in a 6-4 vote. Up to now, most Kansas districts had an "opt-out" policy — they enrolled children in sex ed unless a parent objected in writing.

Only a few other states have such "opt-in" requirements on sex education, according to the Sexuality Information and Education Council of the United States, a group that promotes sex education. Among them: Arizona, Nevada and Utah.

Board members who voted for the new policy said some parents told them they did not know their children were taking sex education until the classes had started.

"It's about empowering parents. That's the bottom line," said board chairman Steve Abrams.

Critics of the measure said that the children whose parents who won't see a permission form or won't turn it in are the ones most likely to need the courses. Some also said that the rule may violate the Kansas Constitution, which gives local school boards broad authority.

In November, after a debate that attracted worldwide attention, the Kansas Board of Education adopted standards that treat evolution as a flawed theory.