Teachers at an Iowa school have created a device that could save the lives of students during a mass shooting.

The invention is called “The Sleeve,” a 12-gauge carbon steel case that fits around the door’s closer arm, securing the door from the inside, according to WQAD.

The Sleeve can withstand more than 550 foot-pounds of force.

It is supposedly nearly impossible to open from the outside.

Teacher Daniel Nietzel at West Middle School in Muscatine got the idea for The Sleeve after participating in the school’s active shooter training.

“We were instructed to tie a belt or a cord around the closer arm. It seemed like a logical way to secure a door without having to go into the hallway, [but] it took us a long time to get a cord, stand on a chair, and tie a knot, which could potentially be the most important tie of your life.” said Nietzel.

“I can tell you in our training, all five rooms that the teachers were trained in; the doors were breached, the cords were ripped, and the officer who was portraying the active shooter came in and killed all of us,” Nietzel said.

In an effort to find a better solution, Nietzel started a company with other teachers, Fighting Chance Solutions, and designed the blueprints for The Sleeve.

After ten months The Sleeve is ready and they are currently waiting for patent approval, reports The Huffington Post.

“We look at it as a cheap insurance policy. If you have someone out in the hallway, and you have an active situation, a dangerous situation unfolding and [don't] want to go out into the hallway and lock your door, we want to provide you with a way to close that door,” said Nietzel.

"My hope," said Edwin Colón, one of the product’s developers, "is to show the rest of our nation that we have to be proactive in addressing this issue that is not going to go away."

“I think it’s a great product. It’s going to buy kids and teachers time for shootings,” said Muscatine Police Chief Brett Talkington. “These shooters [are] going to be pulling on the door. If they can’t get in, they’re going to move on.”

The Sleeve can be purchased individually online for $65, or in bulk, at FightingChanceSolutions.com.