She said 16-year-old Shana Fisher was among the first to be killed in the shooting at Santa Fe High School, which left eight students and two teachers dead and more injured.
Mother Sadie Rodriguez told the Los Angeles Times in private messages that her daughter had embarrassed the 17-year-old suspect in class by finally standing up to him, after putting up with his “aggressive” advances for months.
“A week later he opens fire on everyone he didn’t like,” she said. “Shana being the first one.”
Multiple reports said the shooter focused on the art class where Shana had been, and many said he was taunting victims.
Ms Rodriguez could not independently be reached for comment.
If the allegations prove to be true, it would be the second school shooting in recent months to be associated with such a rejection.
In March, a 17-year-old Maryland high school student used his father’s gun to shoot and seriously wound a female student with whom he had been in a recently-ended relationship, police said.
Investigators have not yet announced an official motive of the shooting, which is the fourth-deadliest to take place at a US school in modern history.
Mr Pagourtzis confessed to the shooting on Saturday, according to police.
Students at Santa Fe High School described him as a quiet loner, who played on the school’s football team.
Dressed in a black trench coat despite the hot Texas weather on Friday, he came to school with a pistol and shotgun for the attack.
He began the attack by firing a shotgun through the window of an art class, according to reports, and lingered for about 30 minutes, during which time he fired more shots.
One witness, student Breanna Quintanilla, was in art class when she heard the shots and someone say, “If you all move, I’m going to shoot you all,” according to Associated Press.
She said Mr Pagourtzis then walked in, pointed at one person and declared, “I’m going to kill you,” before shooting them.
“He then said that if the rest of us moved, he was going to shoot us,” Ms Quintanilla said.
When Ms Quintanilla tried to run out a back door, she realised Pagourtzis was aiming at her. He fired in her direction.
“He missed me,” she said. “But it went ahead and ricocheted and hit me in my right leg.” She was treated at a hospital and spoke with a brown bandage wrapped around her wound.
“It was a very scary thing,” Ms Quintanilla said. “I was worried that I wasn’t going to be able to make it back to my family.”
Mr Pagourtzis’s lawyer Nicholas Poehl said he had not revealed much about the shooting, adding: “Honestly because of his emotional state, I don’t have a lot on that.”
He added that he was investigating whether the suspect endured any “teacher on student” bullying, after reading reports of his client being mistreated by football coaches, but the school have denied anything of the sort.
According to his affidavit, Mr Pagourtzis said he avoided shooting students he liked “so he could have his story told”.
Republican Texas governor Jim Abbott told reporters that Mr Pagourtzis obtained firearms from his father, who had likely acquired them legally.
He added that Mr Pagourtzis had wanted to commit suicide, according to his journals, but did not have the courage to do so.
The suspect’s family said they were “saddened and dismayed” by the shooting and “as shocked as anyone else” as the actions seemed “incompatible with the boy we love”. They said they are cooperating with authorities.
Police said that Mr Pagourtzis was currently being held without bond.
They said homemade explosive devices were also found as part of the investigation, but that they were not capable of detonating.
All schools in Santa Fe will be closed Monday and Tuesday, officials said.
The school’s grief was on display at an evening baseball game where Santa Fe players had crosses painted on their faces and the initials of shooting victims written on tape around their wrists. The team also fashioned a tape cross over the dugout with 10 sets of initials and the words “missed but never forgotten”.
The Santa Fe attack was the 22nd shooting at a school this year that led to someone being killed or injured, according to CNN. It came just months after the shooting in Parkland, Florida which left 17 dead.
The shootings have reignited debate about gun control laws.
Agencies contributed to this report.