Dundee-Crown High School officials said a threat was called in to the school's front office about 9:10 a.m.
After school officials called police, the facility was placed on lockdown.
District officials later said they believed a "swatting" incident took place.
"Unfortunately, swatting has impacted many school districts in Illinois and nationwide. Swatting is illegal. It strains valuable law enforcement resources. It creates unnecessary panic within communities. And, it has significant consequences for the responsible parties," the district said.
In Wednesday's incident, the false call implied a serious threat, according to district officials.
"After a thorough investigation, it was determined that the threat was unsubstantiated, and our students and staff were never in danger," school officials said.
Dundee-Crown High School officials said there was no incident occurring inside or outside the building, but the lockdown was put in place as a precautionary measure.
"I saw people crying and running. It was scary, man," student Alexander Perez said.
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Chopper 7 captured hundreds of students walking out of the school with their hands raised just after 10 a.m.
Carpentersville HS locked down; students seen evacuating
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All students and staff were safely evacuated, school officials later said.
Students stood a distance from the school after the evacuation, and some parents had gathered, as well, upset about the lack of information being shared at the time.
A large amount of armed police from Carpentersville and several other jurisdictions could be seen setting up a perimeter in the area.
"They did a good job of bringing in everybody, this much presence to really evaluate it and see if it was a real threat," parent Jason Rice said.
Some students were seen leaving with parents about 11 a.m., and others were allowed back into the building.
District 300 officials said about 11:30 a.m. that the all-clear had been given.
The school was expected to remain open in a shelter-in-place for the rest of the day, school officials said.
Students would remain in the field house, cafeteria or gym during this time.
The school was expected to dismiss at the normal time, with normal bus services available for students who take district transportation.
School officials said there is a reunification process in place for parents or guardians who would like to pick their child up from school.
Earlier Wednesday morning, school officials said no students or staff were allowed to enter or exit the building, affecting school drop-off.
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One student said as soon as he walked into school for the day, he heard staff calling for a lockdown.
Student describes Dundee-Crown lockdown incident
He described being pulled into a classroom and held there until the classrooms were evacuated one-by-one.
"Right before school started, it was not even 9 o'clock yet. I was just about to go into the bathroom, wash my hands. I come out; I hear the 'Code Red' signs. It's a Code Red lockdown -- go in your classroom, saw a bunch of people yelling. I go to a class; he's trying to get everybody in, yelling at everybody, trying to get everybody in the class, everybody in the corner, lights off, everything," junior Seth Ortiz said.
Code Red is an indication of a possible active shooter in the school.
There were long hugs and lots of tears shared among some parents and their children, after several anxious hours Wednesday morning.
"My heart is finally slowing down. It's been racing," parent Tammy Zilinski said. "I still don't know exactly what happened. I just know that our kids are safe. I'm thankful for that."
Support services will be available for any student who may need them for the rest of the week.
A shelter-in-place was also in effect at Perry Elementary School, due to the lockdown at Dundee-Crown.
"There was no threat at either school during this time," district officials said.
On Monday, South Elgin High School was also evacuated after a "swatting" call.