Stephanie,
First and foremost, teenagers will and do read.
You have lots of data to gather, and then lots of communicating to do.
First, I went through the "all study halls in the library" phase at the high school where I worked. Talk with your principal very honestly about the reasons this step was taken, and its impact on learning (of which book browsing and selection is a part). Are large and supervision-demanding study halls impacting your ability to work with classes in the library, either due to teachers' reluctance to bring their students into a distracting environment, or due to your finding yourself (pardon the word, but we're school librarians here) "babysitting?" Either case would be an instance of lack of understanding of the function of the teacher librarian, or of prioritizing the many hats we wear. I began calling myself a "teacher librarian" around the time I noticed bigger and bigger study halls, and I met with my principal to let her/him know that MY priority was teaching--collaborating with classroom teachers. Your principal may be acting on outdated assumptions of what we do; be prepared to share research studies like
https://www.slj.com/story/research-confirms-value-school-librarians and others. SLJ has published MANY articles on this subject. It is far too easy for administrators to see a big space and what they interpret as a staff member with time on their hands (they don't realize what it takes to collaborate, and that some phases of collaboration look like "just sitting at the computer"), and think they've found the answer to the problem of what to do with students who are not carrying a full schedule of classes, for whatever reason (which is the real problem administrators should be examining when this happens). Let's not make it too easy.
As to the other question of high school students reading, YES, they will read. And here's Part II of your communication: with teachers. One of my favorite repeated collaborative assignments was with a business teacher who found his classes somewhat un-engaged. We sat down together and talked about his curriculum, which included entrepreneurism. We looked at the library's existing collection, then applied for a grant to booster the info on companies and entrepreneurs. We chose companies and individuals that would appeal to 16-year-olds, and focused on shorter, popular, not necessarily YA titles. We tweaked the idea a bit for another of his courses. It became a requirement of the courses that his students would visit the library for a booktalking session from me (so I became familiar with the books, too), time to make careful choices. The end product was a combined presentation about the company/person AND a booktalk for their classmates, which I sat in on and helped assess. One of our Honors/AP social studies teachers required his students to read four outside selections per year, either adult historical fiction (no YA) or a work of popular historical nonfiction (like Devil in the White City). His assignment helped me build my collection in those areas, to the point I shared titles of books the students especially enjoyed with the public library, where some of them went for books (as your students might be doing as well). A few of our freshman English teachers chose to resurrect the quarterly book report, doing a "tasting" for a selection activity. Talk to your teachers. Be open about your concerns that students aren't reading. See where a reading assignment can work in the curriculum. Offer to help. Be ready to tweak the collection, both by weeding and by adding. If you don't already know about it, discover DonorsChoose. There are also collection-development-specific grants from AASL and YALSA.
I haven't forgotten the students here. Form a committee and have them brainstorm books they and their classmates would read if available. Have a suggestion box for titles. And start a book club!!! You'll attract the kids who already like to read. I've found that while we're meeting in the library, they wander the stacks on their way in/out and load up on MORE books. Give it time; it took years to grow my little 3-5 member club into the 50+ behemoth it was the year I retired.
So . . . communicate. And respect yourself and the job you're SUPPOSED to be doing, even if the perception is sometimes "babysitter."
Cathy Andronik
(Retired, Brien McMahon HS, Norwalk)