No charges have been filed as police look at surveillance images from inside the school to determine exactly what happened.
It was parent-teacher conference night at Thornwood High School, but parents say they were told little about what happened.
"It's devastating. But this should be a learning experience for the students," said Toya King, parent, Thornwood High School.
Police say it all began at the school with a fight between two female students, ages 17 and 18. The dispute, students say, was over a pair of designer jeans.
"It was one of them, they owned it, and the girl got mad because she wanted her clothes back," said Jaylan Houston, student, Thornwood High School.
Multiple security guards rushed to get between the girls, one of them was 54-year-old Richard Cowser.
"For whatever reason, he fell into a locker. And once he fell into a locker, he fell down to the floor," said Chief Greg Baker, South Holland Police Department.
Police say Cowser had trouble breathing and was taken to Ingalls Hospital in Harvey, where he was pronounced dead around 1:30 p.m. Thursday afternoon.
"He lost his life. And I think that's just so cruel. I think it's cruel. Kids come to school to learn, not to fight," said Cynthia Fisher, parent, Thornwood High School.
On Thursday night, police were interviewing the girls who got in the fight and awaiting the results of an autopsy scheduled for Friday.
"It appears that no one knows exactly how this gentleman went into the locker, and no one knows exactly how he ended up on the floor," said Chief Baker.
Cowser had been a guard at the school for about a year. His sister told ABC7 he was a body builder and was in good health.
"I just remember his smile. And I just get a picture of him. He was a good man. He didn't deserve it," said Jada King, student, Thornwood High School.
"He was a good security guard. He was a caring person. You would've thought that he had his own kids here," said Fisher.
The security guard's sister says her brother moved here from Florida about a year ago to help out their parents.