As debate rages in Delaware over whether parents should be able to screen school reading lists for "obscene content", the latest list of the books most frequently challenged in US libraries shows it is not only classics that are being challenged.

Books from Fifty Shades of Grey to The Hunger Games have all drawn protests over the last year, with librarians reporting over 300 requests to remove books from shelves or exclude them from school curriculums.

According to local press, a board meeting in the Cape Henlopen school district in Delaware grew heated when two board members started speaking out against Aldous Huxley's dystopian classic, Brave New World, and calling for parents to be warned before children begin studying it. One father is reported to have told the meeting: "Parents might be able to file suit if they felt the school taught obscene content. Why would we teach kids what is negative in society? Let's teach them what is right, to become good citizens and improve the fabric of society."

With the board yet to make a decision, librarians and free speech organisations have lined up to support Brave New World. Deborah Caldwell Stone, deputy director of the American Library Association's Office for Intellectual Freedom, argued that "schemes to label classic works of literature like Brave New World as 'explicit' or 'depressing' only serve to deny students the opportunity to explore the important and challenging ideas presented in these works", adding that "the irony, of course, is that students inevitably seek out and read the books adults ban or label as 'bad books'".

A letter from the National Coalition Against Censorship (NCAC) to the Cape Henlopen school board, meanwhile, warns that "focusing on content that someone might consider inappropriate or objectionable inevitably takes material out of context and distorts the meaning of the book".

"If Brave New World receives a warning, what book will be next, and who will decide which books need warnings?" said NCAC executive director Joan Bertin. "Those who object to this book are entitled to their view, but they may not impose it on others, even to the extent of demanding that the school adopt warnings about content they find objectionable."

The challenges to Brave New World come as the American Library Association (ALA) releases its list of the 10 most frequently challenged books of 2013, after collecting reports on book challenges from librarians and teachers over the past year. A challenge, says the ALA, "is defined as a formal, written complaint filed with a library or school requesting that a book or other material be restricted or removed because of its content or appropriateness".

Topping last year's list was Dav Pilkey's Captain Underpants series, about two schoolchildren who hypnotise their schoolteacher to turn him into a pants-wearing superhero. The bestselling books are, according to challenges, jam-packed with "offensive language [and] violence".

Second was Nobel laureate Toni Morrison's acclaimed The Bluest Eye, which according to one challenge has been demonstrated by "medical and social science" to be "harmful to minors" and is also criticised for the lack of "clear and unequivocal condemnation of the rape, incest, paedophilia, and other repulsive acts" described in the novel.

Sherman Alexie's award-winning young adult novel The Absolutely True Diary of a Part-Time Indian – banned last week in Idaho schools – came in third, with EL James's erotic novel Fifty Shades of Grey, and Suzanne Collins's mega-bestselling dystopian young adult novel The Hunger Games in fourth and fifth places. James's book drew objections over its use of "nudity" and "offensive language", and for being "sexually explicit", while The Hunger Games was challenged for its "religious viewpoint" and for being unsuited to its age group.

The top 10 is rounded out by A Bad Boy Can Be Good for a Girl by Tanya Lee Stone, Looking for Alaska by John Green, The Perks of Being a Wallflower, by Stephen Chbosky, Bless Me Ultima, by Rudolfo Anaya and the Bone comic book series by Jeff Smith.

While the ALA welcomed the fact that the number of reports on attempts to remove material from shelves and school curriculums fell in 2013, to 307 from 464 in 2012, its Office for Intellectual Freedom's executive director Barbara Jones warned against "read[ing] into the decrease in book challenges reported to the ALA, as the removal of just one book from a library prevents hundreds from having free access to information".

Top 10 most frequently challenged books in 2013, out of 307 challenges

Captain Underpants (series) by Dav Pilkey

Reasons: Offensive language, unsuited for age group, violence

The Bluest Eye by Toni Morrison

Reasons: Offensive language, sexually explicit, unsuited to age group, violence

The Absolutely True Diary of a Part-Time Indian by Sherman Alexie

Reasons: Drugs/alcohol/smoking, offensive language, racism, sexually explicit, unsuited to age group

Fifty Shades of Grey by EL James

Reasons: Nudity, offensive language, religious viewpoint, sexually explicit, unsuited to age group

The Hunger Games by Suzanne Collins

Reasons: Religious viewpoint, unsuited to age group

A Bad Boy Can Be Good for a Girl by Tanya Lee Stone

Reasons: Drugs/alcohol/smoking, nudity, offensive language, sexually explicit

Looking for Alaska by John Green

Reasons: Drugs/alcohol/smoking, sexually explicit, unsuited to age group

The Perks of Being a Wallflower by Stephen Chbosky

Reasons: drugs/alcohol/smoking, homosexuality, sexually explicit, unsuited to age group

Bless Me Ultima by Rudolfo Anaya

Reasons: Occult/Satanism, offensive language, religious viewpoint, sexually explicit

Bone (series) by Jeff Smith

Reasons: Political viewpoint, racism, violence