South Central High School in Northwest Indiana held their annual event at the school's gym, the Lake County, Indiana Sherriff's Office said.
About 1,000 people were at the event, which was open to the public.
A concerned parent told police he saw a man openly carrying a handgun on his hip.
A LCSO deputy promptly found the man, and safely took the gun away.
The man was identified as Joshua A. O'dor, 41 of Kingsford Heights, the sheriff's office said.
LCSO said the firearm was a .40 caliber Glock 23 Gen 4.
O'Dor was charged with possession of a firearm on school property. He was released on a cash bond that same night.
"Thank you to the parent for their willingness to report what had been observed," LCSO Administrative Captain Derek J. Allen said. "This is a shining example of 'see something, say something.' We appreciate this type of vigilance to help ensure a safe and secure environment for everyone attending the event."