Woodbridge Vikings Head Football Coach Karibi Dede talks about the first day back at practice after the stabbing death of senior Kenneth Diaz, a member of the football team. (Brad Horn/The Washington Post)

As the school day began Monday at Woodbridge Senior High School, students in red shirts and hoodies got together in a group. They had worn red because it was the favorite color of a classmate who was slain over the weekend.

“One. Two. Three. Kenny,” they yelled, as someone snapped a photo.

Melissa Leassear, a 16-year-old junior, said the students plan to enlarge the image and display it at the funeral for Kenny Joseph Diaz. She said she wasn’t especially close to Diaz but knew him as shy and quiet.

Diaz, 18, was a senior and a member of the football team. He was deeply religious, Leassear said, and would often scold friends for using “the Lord’s name in vain.” A friend remarked to Leassear how he and Diaz would lift weights together. “Now who am I going to lift weights with?” the friend asked her.

Diaz was found Sunday afternoon in Marumsco Acre Lake Park in Woodbridge with a stab wound to the stomach. He later died at a hospital.

Police on Monday afternoon said no arrests have been made. They previously said the attack did not appear to be random and that Diaz may have known his assailant or assailants.

Yovana Villagomez, Diaz’s aunt, said her nephew loved all things sports, especially football, and was a Washington Redskins fan. She said he was an altar boy at Lady of Angels church in Woodbridge and was close to his mother and 12-year-old sister.

In recent weeks, she said, Diaz had been thinking about the future. He planned to go to college, she said. “He was so excited to graduate this year,” she said.

At Woodbridge High, some students cried in the hallways or visited counselors, students said.

“Students and staff of Woodbridge High School are united in sadness at the tragic loss of our friend, teammate and student, Kenny Diaz,” read a message posted on the school Web site. “He will be missed.”

For the school’s football team, the Vikings, the death comes in a year full of promise. Woodbridge has one of the nation’s top-ranked high school prospects, Da’Shawn Hand, who is deciding between three elite college football programs for next year.

On Monday morning, the team began the day with a private meeting in the cafeteria. Diaz, a linebacker, was one of the team’s reserve players.

At 2:49 p.m., a smattering of green and gold appeared on the hill behind the school as the team reported for practice. Vikings assistant coach LeShaun Mack said the captains elected to go with a less-intense “helmets only” practice, instead of hitting in full pads.

Mack said Diaz had missed a recent stretch of practices to focus on his studies, but he returned last Wednesday.

“I said, ‘Where have you been, man?’ ” Mack recalled. Diaz answered him, “I was working on my SATs, Coach.”

Some students said Diaz’s death rattled their sense of safety.

“It just opened our eyes to how sick the world is,” said Valerie Wallace, a 17-year-old senior, who said Diaz was an acquaintance. She and her friend, Amber Lowery, 17, said they had been rethinking many of the things they had taken for granted: walking to friends’ houses or going to the park.

Michael Alison Chandler contributed to this report.