The superintendent provided BuzzFeed News with an image of some of the river rocks that have been collected and stashed in buckets in each of his school’s classrooms.

Before this, Helsel said the district “didn’t really have a plan or program for armed intruders.”

So two years ago, he recruited the help of a company called ALICE (short for Alert, Lockdown, Inform, Counter, Evacuate) that touts itself as the “number one active shooter civilian response training.”

After teachers and students were trained by ALICE on all of the actions to take in the event of an active school shooting, including standardized lock-ins and lockdowns. Helsel thought to implement one “last resort” protocol.

“Every door [to the classroom] has a device installed that secures the door shut,” he said. “It makes it darn near impossible to gain access. Our staff is then trained to barricade the door.”

However, if all else fails, and the shooter attempts to or successfully enters the classroom, the teacher and students will have the rocks to defend themselves.