Free speech advocates in America have slammed a call to ban The Diary of a Young Girl by Anne Frank from schools in Michigan because it contains "pornographic" passages.

A mother of a seventh grader in the Northville school district in Michigan said late last month that Frank's depiction of growing up in hiding as a Jewish teenager during the Holocaust, which has sold millions of copies worldwide, contains "inappropriate material". She pointed in particular to a passage from the "definitive" version of Frank's diary – which includes around 30% of extra material left out of the original 1947 edition by Anne's father Otto – in which the young girl discusses her anatomy.

"Until I was 11 or 12, I didn't realise there was a second set of labia on the inside, since you couldn't see them. What's even funnier is that I thought urine came out of the clitoris," wrote Frank. "When you're standing up, all you see from the front is hair. Between your legs there are two soft, cushiony things, also covered with hair, which press together when you're standing, so you can't see what's inside. They separate when you sit down and they're very red and quite fleshy on the inside. In the upper part, between the outer labia, there's a fold of skin that, on second thought, looks like a kind of blister. That's the clitoris."

The mother told local press that this and other passages had made her daughter "uncomfortable", and that the school should have informed parents about the nature of the material. "It doesn't mean my child is sheltered, it doesn't mean I live in a bubble, and it doesn't mean I'm trying to ban books," she said.

"It's pretty graphic, and it's pretty pornographic for seventh-grade boys and girls to be reading," she said. "It's inappropriate for a teacher to be giving this material out to the kids when it's really the parents' job to give the students this information." She has now launched a formal complaint process asking for the unexpurgated version of the diary to be removed from the school, which is currently under review.

But the district is being urged not to ban the book by the Kids' Right to Read Project, part of the National Coalition Against Censorship, as well as by Frank publisher Bantam Books, the National Council of Teachers of English and PEN America, among others. The organisations have come together to write to the school district, saying that Anne Frank's diary is "both relevant to today's students and pedagogically valuable", and that to "remove the book potentially violates the constitutional rights of other students and parents".

"The passage in question relates to an experience that may be of particular concern to many of your students: physical changes associated with puberty," they write. "Anne had no books or friends to answer her questions, so she was forced to rely on her own observations. Literature helps prepare students for the future by providing opportunities to explore issues they may encounter in life. A good education depends on protecting the right to read, inquire, question and think for ourselves. We strongly urge you to keep The Diary of a Young Girl in its full, uncensored form in classrooms in Northville."

Acacia O'Connor, Kids' Right to Read Project coordinator, added: "Anne Frank's diary is so valuable because it brings students into a world that is at turns very different from their own and extremely familiar. Anne was in fear for her life every day in hiding but at the same time she was experiencing the changes every adolescent faces and her descriptions of those changes are real and important."