(NewsNation) — Police say the 19-year-old gunman who killed a teacher and a 15-year-old girl at a St. Louis high school was armed with an AR-15-style rifle and what appeared to be more than 600 rounds of ammunition.

About 15 minutes passed from the first shot that was fired until the suspected gunman at a St. Louis magnet school was killed in a shootout with police Monday.

During those 15 minutes, students say they texted their parents goodbye, prepared to defend themselves and witnesses as some of their friends were struck by bullets.

Classes were canceled Tuesday at Central Visual and Performing Arts High School, where a former student killed sophomore Alexandria Bell and 61-year-old physical education teacher Jean Kuczka. Several others were injured.

Sophomore Alex Macias said she received strange looks as she and her friend hid in a corner, but they were soon joined by others.

“We heard the gunshots and then we were all piled up on top of each other and the boy in front of me got shot,” she said. “I’m pretty sure he’s still OK, but after the shooter left I made eye contact with him and I thought at that moment ‘This is it.’ My life flashed before my eyes.”

Gunshots could be heard echoing down the hallway in a video that one student recorded from inside a locked classroom.

“I put my phone down and I was looking for something to defend myself with but the gunshots were coming too close,” the student said.

Outside, lines of police cars and dozens of officers flocked to the nearest entrances with guns drawn.

Crowds of children fled the school with their hands up. Others jumped fences.

Kuczka’s daughter says the high school teacher died after putting herself between the gunman and her students when the shooter entered the room.

Former student and now Missouri State Rep. Lakeysha Bosley said Kuczka “easily could have walked away but she stood up for our scholars.”

“She stood up for our kids today and she became a martyr,” Bosley said during a vigil Monday. “No teacher should be a martyr.”

Monday’s school shooting was the 40th this year resulting in injuries or death, according to a tally by Education Week — the most in any year since it began tracking shootings in 2018.

The deadly attacks include the killings at Robb Elementary School in Uvalde, Texas, in May, when 19 children and two teachers died.

Monday’s St. Louis shooting came on the same day a Michigan teenager pleaded guilty to terrorism and first-degree murder in a school shooting that killed four students in December 2021.

The Associated Press contributed to this report.