BRAINTREE — Dressed in pleated plaid skirts and neatly pressed khakis, teenagers from Archbishop Williams High School gathered Monday to mourn the death of their 15-year-old classmate, a loss many described as unthinkable.

In a standing-room-only prayer service at the school’s auditorium, Catholic leaders, school officials, and students remembered Kate McCarthy, a Weymouth teenager killed in a car crash Saturday night. The service culminated a day of reflection and profound sadness, students said, in which academics were secondary.

“It was a lot of sitting around in silence,’’ said James Dolan, a sophomore who knew McCarthy. “Every time someone dies, people say the person lived a happy life, but I can truly say I’ve never seen Kate mad or angry at anyone. She really was always smiling.’’

McCarthy was thrown from an SUV when it crashed into a utility pole on Union Street in Weymouth, then struck a parked car and a tree. Three other sophomores — one girl and two boys — were injured in the crash, according to witnesses and friends.

Authorities have not provided information about their medical condition. Witnesses to the crash said the second girl was also thrown from the car and landed on a neighbor’s porch. She is recovering from her injuries, friends said Monday afternoon. The two boys, one of whom was driving, were less seriously injured, witnesses and friends said.

Police are investigating what caused the crash, and have not named the driver or filed criminal charges. A neighbor who was awake during the crash said she heard an engine revving before speeding down the street, but other neighbors did not hear the car before the crash. Authorities have not indicated whether speed was a factor.

Sarah McLoughney, a sophomore at Archbishop Williams, left the prayer service with her mother, and wept while holding a single white rose. McCarthy had been like a sister to her, she said.

On Thursday night, she said, they had a sleepover at McCarthy’s house, where they watched the coming-of-age movie “Marley and Me,’’ sang their favorite songs, and made concert plans for the summer.

“She kept talking about the beach,’’ McLoughney said. “She was just a great friend.’’

McLoughney asked people to remember McCarthy’s beautiful smile and her hard-working nature.

Kay McLoughney, Sarah’s mother, said the teenager’s death was a “nightmare,’’ and that it was heartbreaking to see her children’s friends experience such tragedy at a young age. “They all looked up to her,’’ she said.

McCarthy was a standout student, sang in the school chorus, and played three sports — volleyball, lacrosse, and track and field.

Anthony Agnitti, a board member at Archbishop Williams and a friend of the McCarthy family, said the prayer service was only the beginning of the school’s grieving process. Both sides of McCarthy’s family are “well known and well liked’’ among the school’s parents and teachers, Agnitti said.

“They go to church, they give back to the community, and they’re always helping others,’’ Agnitti said, his eyes welling with tears.

Dolan, the Archbishop Williams sophomore, said he visited the teenage driver Sunday, and that he was taking McCarthy’s death “the hardest’’ way imaginable. “He just keeps blaming himself,’’ Dolan said.

Ted Ma and Hans Zen, seniors at Archbishop Williams, said students are in disbelief over the fatal crash. Ma called it “shocking,’’ and Zen said he was still attempting to process what happened.

“I didn’t believe it when I heard,’’ Zen said. “It’s crazy that people who are around can just disappear.’’

At the crash site, students wrote messages of condolences on the newly replaced utility pole, and stared at the lingering evidence of the crash, including a divot on a driveway and leftover glass from a shattered windshield.

“RIP Angel. You will forever be missed. You were so beautiful inside and out,’’ wrote one friend. Others asked McCarthy to “watch over’’ and protect her friends from above.

McCarthy would have turned 16 in July, Sarah McLoughney said. At Thursday’s sleepover, she was already beginning to plan a special Cape Cod party for friends.

Astead W. Herndon can be reached at astead.herndon@globe.com. Follow him on Twitter @AsteadWH.