Authorities got a 911 call around 9:44 a.m. saying an intruder was inside the west suburban middle school.
The female student was taken to an area hospital while police put the campus on lockdown.
Access to a nearby grade school was also restricted as a precaution.
Several tense hours passed with no one suspicious found in the school building.
"We need to check all the video, and we haven't done that," said Bill Hayden, Addison Police Department. "The investigation is continuing right now ... every sign-in sheet, every video camera, every door is being checked."
So far, investigators have refused to share details about exactly what happened as school administrators tried to reassure parents about safety.
School officials will have counselors on hand on Tuesday; Monday is a holiday.