Police identified the suspect as 18-year-old Jakob Wagner. He used to attend Antigo High School, police Chief Eric Roller said.
The victims, who were shot as they left a building where they had been celebrating prom, suffered injuries that were not life-threatening, according to police.
A female victim was treated for a gunshot wound and released, while a male victim was undergoing surgery late Sunday morning.
About 11 p.m. Saturday, Wagner "arrived at the Antigo High School prom with a rifle and began shooting while outside the entrance of the building," the police statement said.
Officers who were patrolling the parking lot heard the shots, and one of the officers opened fire on the gunman. Wagner was taken to a nearby hospital, where he died at 1:06 a.m. after doctors attempted lifesaving measures, the statement said.
Wagner is believed to be the lone shooter, police said, and investigators were executing a search warrant at his home Sunday.
"All the people within the prom were eventually escorted from the school and are safe," Roller said.
Sonia Reed, whose son Matthew attended the prom, told WSAW
that she was on campus earlier in the evening for the "grand march," during which students and their dates form a procession and parents cheer them on before the prom king and queen are named.
She estimates that there were 100 to 120 students in attendance and describes the atmosphere as "festive."
"I didn't see anything suspicious. I didn't feel any bad vibes. It seemed like it was going to be a normal prom," she told the station.
She learned of the incident, she said, when she received a call from another mother who said police were at the school and had "somebody down" in front of the school offices. The person was wearing camouflage pants and a dark top, Reed said. Her son had already left the prom by that time, she said.
Reed, who moved to Antigo from Texas in 2004, said that although she wouldn't be surprised to hear something like this had happened in a major city, she was stunned it unfolded in Antigo.
Everyone knows each other in the town of 8,000 about 85 miles northwest of Green Bay, she said. She described it as a farm town where many students happily participate in Drive Your Tractor to School Day.
"I have two other children that's in the high school, and they don't want to go back, period," Reed said. "They're just beyond freaking out here."
School officials haven't released many details about what unfolded Saturday night, citing the pending investigation. But in a statement posted on the school district's Facebook page Sunday, they pointed out that the gunman never entered the building because of the police presence at the event.
"Counseling services will be available in all schools tomorrow, Monday, for students and staff who may have been traumatized by this terrible incident," the school district said. "We are thankful police and staff acted quickly together to prevent this incident from becoming a major tragedy."