A Lakeside High School student, a driver in a Saturday crash that killed Leah O’Brien, one of his teachers at the DeKalb school, was arrested Thursday afternoon.

Decatur police say charges against Ramiro Pedemonte, 19, of Tucker include first-degree vehicular homicide and reckless driving. He was arrested at his home without incident.

O’Brien, 33, was headed to chaperone Lakeside High’s prom when her Hyundai Sonata collided with Pedemonte’s Dodge Charger, as he and his date, a fellow student, were headed to the same prom.

“(O’Brien) was always at the prom,” Lakeside Principal Jason Clyne told The Atlanta Journal-Constitution. “She was one of the regulars.”

Clyne added that the students were unaware that they had collided with their own teacher until the next day.

Clyne said O’Brien was a natural mentor and sometimes worked with students who were headed the wrong way in life. Volunteering at the prom was, for her, a way to see her efforts pay off — to see these kids dressed up and behaving well, he said.

“When you see troubled kids at the prom, when you see them put on a tux, you realize they do know how to act right, and it warms our hearts, ” Clyne said. “She was looking forward to that moment.”

O’Brien, who had undergraduate and graduate degrees from Hampton University in Virginia, taught family and consumer science at Lakeside and was the faculty adviser for the Purple Diamonds dance team, which performs at basketball games and other sporting events.

“She’s a great motivator, ” said Georgia Collins, the coach of the dance team and a bus driver for the school. Collins said O’Brien organized members of the dance team to participate in a walk-a-thon to raise money for St. Jude, where Collins’ son Archie was treated for lymphoma in 2012.

“We wound up having a very special bond, more than just co-workers, ” Collins said.

Collins said Pedemonte was in one of O’Brien’s her classes.

“He was very close to her,” Collins said.

O’Brien has two daughters at Fernbank Elementary, Kori and fourth-grader Rachel. Rachel was being cared for by family members, according to school officials.

A candlelight vigil will be held Thursday evening to celebrate O’Brien’s life.

The vigil is scheduled for 8 p.m. at Lakeside’s track and will feature a tribute performance by the Purple Diamonds Dance Team, as well as farewell walk.

An education fund to provide for her two daughters has also been established, "Leah's Lil Angels." The fund can be reached online at www.gofundme.com/Leahslilangels.

A viewing for O’Brien will be Friday from 10 a.m. to 8 p.m. at Gregory B. Levett & Sons Funeral Home/South DeKalb Chapel, 4347 Flat Shoals Parkway, Decatur.

The funeral will be at 1 p.m. Saturday at Saint Philip AME Church, 240 Candler Road SE, Atlanta.