30 seconds to make a decision? If armed pull your weapon and put him down would be the perfect answer however the odds of him being armed are slim to none because carrying on school property is currently against the law. Lock it down, call 911 and have plan in place to get the kids out of the back and into a safe place.
http://www.thebrunswicknews.com/open_access/local_news/SAFETY-932013-hr-GJ#
http://www.thebrunswicknews.com/open_access/local_news/SAFETY-932013-hr-GJ#