GILBERT, Ariz. — The textbook, the one with the wide-eyed lemur peering off the cover, has been handed out for years to students in honors biology classes at the high schools here, offering lessons on bread-and-butter subjects like mitosis and meiosis, photosynthesis and anatomy.

But now, the school board in this suburb of Phoenix has voted to excise or redact two pages deep inside the book — 544 and 545 — because they discuss sexually transmitted diseases and contraception, including mifepristone, a drug that can be used to prevent or halt a pregnancy.

A law passed two years ago in Arizona requires schools to teach “preference, encouragement and support to childbirth and adoption” over abortion, and the school board decided that those pages were in violation of this law — even though the Arizona Education Department, which examined the book for compliance, found that they were not.

The controversy has turned into a referendum on the 2012 law, with supporters saying the textbook content cannot be removed fast enough and opponents crying foul for any number of reasons: technical, ethical, pedagogical. But the Gilbert school board is moving forward, trying to figure out how to remove the material in question — by way of black markers or scissors, if need be — despite resistance from parents, residents, the American Civil Liberties Union and even the district’s superintendent.