Vandalism in the library

I might have mentioned this before in previous entries, but my library is located next to a middle school. As such, a lot of the kids come to the library after school. Wednesdays are a particular problem because the schools in KCK let out at one o'clock so that teachers can participate in training sessions or work on other things. We are having a problem with the teenagers being unruly and trying stunts when our security guard is off duty. Last night, we had two teenage boys trash one of the men's bathrooms and also throw trash around one of our OPAC computers. Security camera shots showed that these boys were in the area of the scenes of the crime. Unfortunately, even though every staff member knows these boys did indeed trash the bathroom and computer areas, our evidence is only cirmcumstantial.
Today at work, I printed off the pictures of the scenes of the crime and submitted them to both the security as well as the library branch manager. My mind is swimming with ideas on how we can deal with these teenagers but unfortunately these ideas would only put a minor dent into the problem.