...before you assume 'adult books' means stuff you find in an 'adult bookshop' - nah. Clicking about in relation to Soulless by Gail Carriger (huge fun, one of my Easter holiday reads) ...
I tripped over the American Library Association's Alex Awards - recognising books that were written for the adult market but have found eager readers among teenagers and young adults. While there's a tad of an American skew (hardly unsurprising), it's still a handy little collection development tool. There are links on the pages to the Alex Awards in 2009 and earlier years, so you can hunt back through for possible additions to your school library.
Another on the 2010 list is the dark graphic novel Stitches: a memoir by David Small, which I bought on the recommendation of Paul McDonald of the Children's Bookshop in Beecroft.
I don't have a separate 'senior' section or stickering (kids in my school are years 7-12, or about 11-18 years old); what I do for the occasional book is have a note in the back next to the date due slip (where whichever staff member is lending out the book will see it) with a wording that goes something like, "Dear Reader, if you are in Years 7,8,9 or 10, you will need to consult the teacher librarian before borrowing this book." I figure this means we can chat about those which may need chatting (eg. the Year 7 boy who wanted to borrow The Time Traveler's Wife on the basis of the time travel bit, rather than realising quite the marital detail involved in the wife bit). It also means those in search of titillation (hey, a Senior Book - it must have Good Bits!!!) have a hard road to hoe, as they don't have external cues (separate section or stickering) to help them on their way. If a junior kid is dead keen to borrow a book with this note in it, one option is to phone the parents/guardians and discuss the matter, and this usually ends up being a productive road to a solution.
Two novels I've read in the last couple of years which fell on the Yup and Nope sides of my collection development: although Love like water was a shortlisted book in the Children's Book Council Awards in the older reader category a couple of years ago, it has been borrowed not much if at all. I read it, and thought the age of the main characters (around their early twenties) and their activities and preoccupations made it much more of a book interesting to young adults rather than high school age kids; I wasn't brought to care much about the characters or what happened to them. I've included it in displays, had it on my face-out book stand and so on, but it just doesn't find friends. If I'd read it before buying (and let's face it, you can't read everything before buying), I don't think I would have. So it's a Nope. Last hols I read Melina Marchetta's new book, The piper's son, a few-years-later sequel to Saving Francesca. I'm much more confident that this will find readers, and not only because it's a sequel. While it too is suited to older high school students, it's more engaging in its storytelling, drawing you in and along. The characters are in their early twenties, but more accessible and engaging, the focus of the book more positive than the underlying negativity I felt while reading Love like water. I read the Marchetta book before adding it to the collection, wanting to be aware of its content, and it's a definite Yup.
image source for Soulless
image source for Stitches