Zeina Alva, 45, was a longtime custodian at the school.
Police say at this point they have no reason to believe there was any foul play involved. They say it was either an accident or possibly a suicide.
Alva's body was on the foundation floor of an addition to the main school building that is currently under construction. Naperville police say an ironworker first saw the body early Monday morning. His supervisor then called police. Some students apparently saw the body as well.
The school put information on its Web site and later made an announcement on the intercom.
"I didn't even know about it until fourth hour and then all of a sudden everyone knew," said Shannon Rueb, student.
"An announcement came on and they didn't say a name or who it was. Then my sister texted me and said it was one of your custodians," said Eric Harle, student.
School officials initially declined to identify the women to students. After police released her name, word filtered out and the school's principal released a statement that said, "the Naperville Central High School community was shocked and saddened by the loss of longtime staff member who was loved by staff and students alike and will be dearly missed."
Alva worked at the school 16 years and was a familiar face to many students.
"She was a really nice person. I'm really sorry she's gone," said Neel Kunjur, student.
"We heard about the body and everything and then after lunch, when I found out, I was just, like, you know, I didn't see that coming. It's a shame. She was always there for the students and she was really nice," said Nevin O'Sullivan, student.
Police are calling it a death investigation. They are awaiting autopsy results which they hope will help them determine both the cause of death and possibly a time of death. At this point, they are still unsure how she gained access to the construction site, although she did have credentials to get into the school.