The Texas gunman had few friends in high school, classmates say.

The gunman who fatally shot 19 children and two adults in a Texas elementary school was a high school senior who frequently missed school and struggled to get along with classmates, acquaintances said on Tuesday as they tried to make sense of the massacre.

The 18-year-old gunman, Salvador Ramos, who died at the scene, had few friends at Uvalde High School, his classmates said.

Jeremiah Munoz was a senior at the high school four years ago when he bonded with Mr. Ramos, then a freshman, over their shared love of video games, particularly Fortnite and Call of Duty. Mr. Munoz, 22, said that even back then, he recalled students picking on Mr. Ramos, deriding his clothes or making crude references to his mother or sister.

Charlie Marsh, a 17-year-old in the same grade as Mr. Ramos, said that she had heard people call him names, including a homophobic slur, but that she thought he was provoking people rather than being bullied.

Over the weekend, Mr. Ramos had sent a photograph of two black rifles to Mr. Munoz, similar to one he had posted on his Instagram account.

Graduating seniors from Uvalde High School had visited the elementary school on Monday and high-fived the students, who are in second through fourth grade, as part of the high school’s senior week. One student who participated said on social media that Mr. Ramos had not attended.

Mr. Munoz said that when he played Xbox with Mr. Ramos, he would often hear him arguing with his mother through the microphone. Mr. Ramos’s mother would scream at him, telling him that he needed to go to school and that he was doing nothing with his life, Mr. Munoz said, and Mr. Ramos would yell back, calling her expletives.

He would often leave his mother’s home and stay with his grandmother for several days after a big fight, Mr. Munoz said, and in the past year it seemed that he was spending even more time at his grandmother’s home. The authorities have said that Mr. Ramos’s grandmother was shot before he crashed a pickup truck near the elementary school, which was about a half-mile from her home, and killed the children and adults.

Several people who followed Mr. Ramos on Instagram said they had first met him on websites designed to connect strangers, including Yubo, an app that lets people livestream videos of themselves and markets itself as a way to “get friends.” One woman said she had connected with Mr. Ramos and a friend of his on Omegle, a website where people video chat with strangers. The woman said he had once livestreamed himself holding a gun with blood visible on the floor, claiming that he had a nosebleed.

Ms. Marsh said that Mr. Ramos had hardly shown up to their shared fourth-period class this year, but that she saw him working at a Wendy’s in town about two months ago. The night manager there said that Mr. Ramos went out of his way to keep to himself.

“You know how my guys talk to each other and are friendly?” Adrian Mendez, the night manager, said on Tuesday evening as he motioned to two employees. “He wasn’t like that.”

Edgar Sandoval contributed reporting from Uvalde, Texas.