Get a display space near the cafeteria or some other place where students go frequently. Put books there that are interesting to the students. Thor comics, Ender's game whatever the media is already advertising for you.
Talk to teachers and hold classes in the library occasionally so the kids feel comfortable there.
See if the school will add DVDs to the library's collection.
Get them there and they'll figure out how to use it, but you have to get them there.
If they can put meeting rooms in, so clubs can meet there that would be great as well.