The fatal fall occurred about 5:25 p.m. at the school, 5330 S. 1660 East. The boy's name was not released.
"The [boy's] mom was just screaming that tearful pain," said Gary Sanchez, whose mom's home is in the quiet neighborhood across the street from the school. He heard her wail from about 100 yards away when a paramedic told her the news.
The teen, who did not attend the junior high or Granite School District, scaled the school with another 16-year-old boy, said Salt Lake County Sheriff's Detective Shane Manwaring. They got onto the roof possibly from the south side. The Holladay teen continued up onto the gym roof while his friend waited below.
A soccer coach driving into the northwest parking lot spotted the Holladay teen near the edge of the roof and the two made eye contact, Manwaring said. The Holladay teen ran. Seconds later, his friend heard a crashing noise.
The friend called the Holladay teen's name but got no response, Manwaring said. He then climbed onto the gym roof, saw a broken skylight and looked down. His friend was lying on the gym floor 40 feet below.
Sanchez's mother, Kathy, was sitting outside under a tree when the Holladay teen's friend rode over on his bike from the south side of the school and said his friend had fallen through the skylight. Kathy Sanchez said the agitated boy told her his friend wasn't moving and he might be dead.
"He said, 'Should I call his mom?' and I said, 'Yes,' " Sanchez said.
Kathy Sanchez called 911 and then went to the back of the school with the boy and waited for paramedics and police to arrive. Soon, the Holladay teen's mother arrived, as well as his father and three other boys.
"I stayed with her and prayed with her," Kathy Sanchez said.
Yellow police tape flapped in the wind near the gym's north door as detectives investigated and paramedics tried to resuscitate the boy.
About 20 minutes elapsed before a paramedic returned. "The paramedic said, 'I'm sorry, he didn't make it,'" Kathy Sanchez said.
The Holladay teen's mother burst into tears along with the other family members.
"The dad and the boys were just so upset," Kathy Sanchez said.
Manwaring said the gym was empty when the teen fell through the roof. "It's a tragic accident," he said.
Soccer players using the field west of the school's gym continued their practice as police finished their investigation, at about 7:45 p.m.
"This is something no parent should have to go through," said Granite School District director of communications Randy Ripplinger.