John and Maria Castillo are now leading advocates for arming teachers

• "We are the living example," John Castillo says, of what happens when a school shooter is not confronted by an armed person.
• "You're going to have an Uvalde, you're going to have Columbine — these things are going to happen again and again until we do something that is tangible and something that makes a difference," he told Axios in a recent interview.

Flashback: Kendrick, 18, charged a gunman who burst into his STEM classroom in 2019 and helped subdue him before being fatally shot. Other students wrestled the gun away and prevented further bloodshed.

What's new: The tragedy transformed the Castillos into leading advocates of one of the most controversial responses to school shootings.

• The parents testify to legislative committees and urge school boards to allow teachers to carry firearms in schools. They also support FASTER Colorado and Bullets Both Ways, a clothing brand that sponsors armed teacher training.
• He wants armed teachers to be as common as air marshals on planes and believes the Uvalde shooting "is a tipping point" in the debate.

Between the lines: Before he committed to the cause, John Castillo completed the first level of training for teachers to ensure it "had value to it," he says.

• In the course, he kept his presence a secret and struggled privately as instructors showed Kendrick's photo and debriefed recent school shootings.
• "Emotionally, it was tough," he recalled. "And it was also very cathartic in a way."

Zoom in: Now, he speaks to most FASTER Colorado cohorts, retelling his son's story and his journey in a way that makes the entire training more real.

• Over lunch at the recent training, Castillo wore a T-shirt with his son's face on it and thanked the school personnel in front of him for bearing the responsibility that comes with being armed.
• "People have such misconceptions about what a true protector is. They think we are all just gun-toting, crazy people that want to kill for no reason," he told them. "They don't realize you are the last level of defense in an act of domestic terror."

The other side: Castillo knows his advocacy alienates others. He knows what opponents say and the deeply embedded politics in the idea of arming teachers.

• In response, he just recalls his son's death: "Our children are having to make decisions to save their own lines, so I just ask people to keep an open mind."

Related: Colorado schools arming more teachers after Uvalde shooting ... Schools turn to Colorado-made tech to prevent shootings