From the letter:
These and other reviews attest to the literary and educational value of the book. In contrast, no legitimate pedagogical rationale has been advanced for its removal, and it is highly doubtful that any legitimate justification could be advanced, especially for removing the book from the library, the purpose of which is to give students the opportunity to explore books on their own, according to their own interests, views and values.
See also: the related press release.
See also: an article from the school newspaper:
After a group of students noticed the cover of David Levithan’s 2013 novel, Two Boys Kissing, parent Jessica Wilson launched a book challenge to remove it from FHS’s library. The complaint was officially filed on the grounds that the picture on the book’s cover, which features two boys kissing, violated the school’s policy of no public displays of affection. Furthermore, Wilson was concerned that the book had overt sexual content.
In that article, there's a quote from the challenger:
“The good thing about appealing is that it opens the matter up to public debate,” Wilson said. “It’s not like this isn’t a book that I wouldn’t let my kids read, but it’s the fact that it’s in a school. Books like The Scarlet Letter and One Flew Over the Cuckoo’s Nest don’t embrace sexuality. They have consequences, and it’s integral to the story. When you’re a teenager, it’s normal to question your sexuality, your faith, but the school isn’t your nanny; it isn’t up to the school to provide this guidance.”
I'm fascinated by her logic here: she says that the school "isn't your nanny" and that it isn't up to the school to "provide [this] guidance", but it seems to me that in asking for the library to only include stories in which sexual contact has "consequences", that's EXACTLY what she's asking the school to be and to do.