"It was going to be a good day," Reyes told ABC News anchor Amy Robach in an exclusive interview. "There was nothing unusual that day, we were just walking back to the classroom ... to watch the rest of the movie."
Around 11:30 a.m., however, the normalcy shattered. Reyes said he heard a bang. Unsure of what it was, he told the students to get under their desks -- just like they'd practiced.
"The kids were yelling, 'What's going on, Mr. Reyes?'" he said. "[The students] were going under the table, and I was trying to get them to do that as fast as I could."
"When I turned around," he said, "I just saw him."
WATCH: How law enforcement narrative of Uvalde mass shooting changed
The next 77 minutes of carnage "destroyed" Reyes, he said, and forever changed a school, a community and perhaps a country. By the end of his rampage, a gunman had killed 19 students -- including all 11 in Reyes' classroom -- and two teachers. Reyes himself sustained multiple gunshot wounds.
"I feel so bad for the parents because they lost a child," Reyes told Robach. "But they lost one child. I lost 11 that day, all at one time."
From his hospital bed in San Antonio, less than two weeks after surviving the second-most deadly school shooting in U.S. history, Reyes offered the most vivid account yet of what transpired inside classroom 111 of Robb Elementary School on May 24.
MORE: What we know about 21 Uvalde school shooting victims
He also waded into the nationwide debate over gun violence and slammed local police as "cowards" for failing to act faster. And while Reyes recovers, he's already plotting his next act: ensuring this never happens again.
"The only thing that I know is that I won't let these children and my co-workers die in vain," he said. "I will go to the end of the world to make sure things get changed. If that's what I have to do for the rest of my life, I will do it."
SEE ALSO: Video appears to show Texas 911 dispatchers relaying info from kids in Robb Elementary classroom
In the wake of the shooting, law enforcement has come under immense scrutiny for failing to act faster. Seventy-seven minutes passed from the time the gunman entered the school until officers breached the door and killed him.
"They're cowards," Reyes said. "They sit there and did nothing for our community. They took a long time to go in... I will never forgive them."
Law enforcement and state officials have repeatedly corrected themselves and at times provided conflicting details about their response. At one point, a Texas Department of Public Safety official said the on-scene commanding officer made the "wrong decision" to wait to breach the barricaded classrooms.
WATCH: Official says police made 'wrong decision' not to enter classroom
Robb Elementary School prepared for active shooter events, conducting drills as recently as a few weeks before the mass shooting. But Reyes described failures in protocol at nearly every step of the security process on May 24 -- missteps in protocol and execution that he says cost lives.
"There was no announcement. I did not receive any messages on my phone -- sometimes we do get a Raptor system," he said, referring to the school district's emergency alert program, "but I didn't get anything, and I didn't hear anything."
Reyes also described complaints he said he had made about his door, which is meant to remain shut and locked while class is in session. At prior security checks, Reyes said he noticed that his door would not latch -- an issue he said he raised with the school's principal.
"When that would happen, I would tell my principal, 'Hey, I'm going to get in trouble again, they're going to come and tell you that I left my door unlocked, which I didn't,'" he said. "But the latch was stuck. So, it was just an easy fix."
MORE: Uvalde police forces no longer cooperating with school shooting probe, source tells ABC News
Even with the failures in plan implementation, Reyes said the outcome felt inevitable: "No training would ever prepare anybody for this."
"It all happened too fast. Training, no training, all kinds of training -- nothing gets you ready for this," he said. "We trained our kids to sit under the table and that's what I thought of at the time. But we set them up to be like ducks."
The Uvalde Police Department, Texas Department of Public Safety and Robb Elementary School did not respond to requests for comment.
The solution is not more training, according to Reyes, but an overhaul of a system that allows easy access to firearms. Reyes emphasized that he is not against gun ownership, but advocated for common-sense gun legislation that would raise the age limit for would-be gun purchasers.
"If you want to buy a gun, you want to own a gun, that's fine," he said. "But the age limit has to change. And I think that they need to do more background checks on it. Things just have to change. It must change."
SEE ALSO: Mass shootings in the U.S. have nearly tripled since 2013, data shows
The number of people injured or killed does not include the suspect or perpetrator. These graphics show the number of victims across all mass shootings from the last five years.