In my experience as both a classroom teacher and an elementary school librarian, I have found that what different families consider "sensitive" or "inappropriate" varies greatly.  I do my best to keep our library collection both appropriate for the children's maturity levels as well as uncensored so that they do not miss out on access to excellent literature.  I have had one family ask me not to let their children check out graphic novels and another beg me for them.  Creating a law that requires marking sensitive material will be both daunting and pretty much impossible since someone can usually find something they deem "inappropriate" in many many books.  This proprosal leans more towards censorship rather than helping students.  Parents should definitely be involved in helping their children choose, read, and understand great literature, but this is not the way to do it.