This is a *school* library, not a public library. The distinction is important.  I worked in a public library (in the children's department) for almost 5 years and I wholeheartedly agree with you on the topic of censorship.  One patron-- even a group of patrons-- should not be able to dictate what the entire community can or cannot read.  

However, books in a public library are shelved according to reading level: children's, young adult and adult.  This is an elementary school library where the patrons are around 5-11.  There's absolutely no question that the graphic novel was "age inappropriate" and even Marvel recognized that by labeling it for ages 12+.  At the very least, the librarian should have checked the age level and refused to let the 6 year old check it out. (Much like public libraries will do with adult books or R rated movies.)