When I took the June 14th LSAT, the proctor walked around at the last minute and snagged a couple bottles of water from people. I didn't see anything else taken, just water. I had a small bag with Kleenex, pencils, etc. and I think you'll be fine if you keep it out and aren't digging through a backpack during the test. You should be able to call ahead to the site and see what specific rules they have.