A Pennsylvania school district is arming classrooms with buckets of rocks as a last-ditch defense against mass shooters. David Helsel, superintendent of the Blue Mountain School District in Schuylkill County, told ABC News on Friday that every elementary, middle and high school classroom in the district is stocked with a 5-gallon bucket full of river stones for students and teachers to pelt an armed intruder. “We’ve been trying to be proactive just in case,” Mr. Helsel said. “How can you aim a gun if you’re being pelted with rocks?”