The Schofield Middle School pupil, who underwent a heart transplant in 2004, died Monday at the Medical University of South Carolina in Charleston, S.C.
Haley's health began failing a little more than a month ago, said her father, Adrian Baer.
Last Thursday, Haley's family found her unconscious in her bedroom, her grandmother, Debra Baer, said.
"There were no grave indicators. It's still a shock to us," she said. "We still don't know what happened, and it'll be a month or more before we do. That's the tough part."
In December, the lively 14-year-old first came to terms with death, Debra Baer said. Haley underwent a routine medical procedure, and her heart stopped.
"That was the first time she started contemplating that she could die," Debra Baer said. "It was very intense. She was still happy, but, in her quieter moments, she would have more questions."
Her family jokingly said she majored in socializing, and lived for outdoor events such as water skiing and swimming. She also had a passion for gymnastics and participated until she had the heart transplant, made necessary by a congenital defect, when she was 9.
Although her classmates are in a state of mourning, Principal Carl White said, her lasting impression is one that's celebrated.
"She was very popular," he said. "She was a student who loved her friends, so much so that even when she wasn't physically well, she was here."
By Wednesday afternoon, a Facebook page -- RIP Haley Baer, started by classmate Callie D'Andria -- already had more than 700 members. Many of the comments were from strangers who appreciated the kind words of the people who knew Haley.
Debra Baer said a collage created by Haley's friends after her death characterizes the way she lived her life.
"It reads, 'Don't be afraid of dying, live a life worth dying for,' " she said.
The family will hold visitation Friday from 5:30 to 7:30 p.m. at the Shellhouse Funeral Home, located at 924 Hayne Ave. in Aiken.
Haley's funeral will be held at 11 a.m. Saturday at St. John United Methodist Church in Graniteville. Classmates will serve as honorary pallbearers.