School Bomb Threats: No Laughing Matter
Schools have been plagued by fake bomb threats for decades. However, due to several real-life violent attacks on schools in recent decades, what used to be treated as an ill-advised prank and subject to administrative actions like suspensions and expulsions can now result in criminal charges and jail time.
A real bomb is always a possibility.
It isn't hard to make a bomb. Books like The Anarchist Cookbook, published in 1971, tell you how and are available through public libraries.