According to a letter from District 113 Superintendent Bruce Law sent to families, the district received an anonymous tip just after 10:20 a.m. of a rumor of a student with a gun.
"After what happened in July, everyone is on high alert here always. I got here, and my stomach sank," said David Fettner, the parent of a Highland Park High School student. "It was like a flashback to July."
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Highland Park High School officials reported the tip to law enforcement, and another student came forward reporting to have seen a student with a gun. Highland Park police then asked the school to lock down.
Police cars bolted to campus. Officers covered head-to-toe in ballistic gear and armed with high-powered rifles entered the school while others scoured the school's roof.
WATCH: Heavily armored officers seen inside Highland Park HS amid lockdown
Heavily armored officers seen inside Highland Park HS amid lockdown
The student who allegedly had the gun was identified, and security footage showed he and another student had left the building, the letter said.
Five students have been taken into custody. The lockdown was lifted shortly before 12:30 p.m., and students flooded out of the school.
"I just really feel for all of the kids who can't come to school on a daily basis and know that they are going to be safe," said Michael West, a Highland Park High School graduate.
WATCH: Law enforcement officials file into Highland Park HS amid lockdown
Law enforcement officials file into Highland Park HS amid lockdown
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Other public buildings, including city of Highland Park buildings, District 112 schools, District 113 schools, the Park District of Highland Park and the Highland Park Public Library, also had their lockdowns lifted.
Lake County Clerk Anthony Vega said some voting had to be stopped at the other public buildings.
"You think you're in a safe neighborhood, and you find out this is happening in your school, and you find out the school isn't as safe as you thought it was," one parent said.
The lockdown went into effect just minutes after students at Highland Park High School had walked out, calling for an assault rifle ban.
Highland Park HS students walk out over gun safety
The action was in response to last year's July Fourth parade shooting and the recent school shooting in Nashville, Tennessee.
The teens planned to leave class at 10:14 a.m. That's when gunfire was first heard during the Highland Park parade.
RELATED: 1st funeral for Nashville school shooting victim held; police release terrifying 911 calls of attack
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They're demanding a ban on assault weapons.
Thousands of students across Nashville walked out of class Monday, exactly one week after a deadly shooting at the Covenant School.
RELATED: 'This is gonna keep happening': Highland Park survivor heads to scene of Nashville shooting
Students marched to the Tennessee state Capitol to call for action on gun violence.
The state's governor announced new funding for school security Monday, including $140 million to hire school resource officers.
Highland Park Mayor Nancy Rotering said in a statement in part:
"While we were so relieved that the event ended without incident, once again, our community was terrorized by the threat of gun violence. I urge all affected residents to reach out for trauma support by calling 211. 211 provides free and confidential multilingual assistance from trained navigators who can connect you with local available resources to manage trauma 24/7. Visit 211LakeCounty.org, call 211, or text your zip code to 898211.We must continue to work towards a time when we can live free from the threat of gun violence. Those who can, need to take action now, so that our students can go to school without fear of random gun violence. Wherever and whenever they occur, every lockdown, every drill takes a toll on our children and our communities."
Vega said the Braeside Elementary School, Red Oak Elementary School, Highland Park Public Library voting sites impacted by the lockdown will be open until 8:35 p.m. The JIC Community Development Center will close at 7:40 p.m.