Officers who were outside the Marinette High School classroom said they heard three gunshots shortly after 8 p.m. and busted through the door, said Police Chief Jeff Skorik said. The 15-year-old male gunman then shot himself.
Skorik said the teen was taken to an area hospital and his condition was not immediately known Monday night.
Officials earlier had said there were no reports of shots fired, but student Zach Campbell said he and his classmates had been watching a film about Greek myths at the end of the school day when the hostage-taker pulled out a gun and shot the projector. He then fired another shot.
"It was a very scary event," Campbell said. The teen made students put their cell phones in the middle of the room and broke his own phone when it rang. The class then spent six hours talking to him about hunting and fishing.
"We just wanted to be on his good side," Campbell said. He said the gunmen seemed depressed. "But he didn't really seem like he wanted to hurt anybody."
Campbell was among five students who were released about 20 minutes before police entered the classroom after convincing the gunman they had to use the bathroom.
The other 18 students and the female teacher, who had acted as a mediator between the male hostage-taker and authorities, were released unharmed once officers were inside.
Skorik said the gunman had refused to communicate with officials during the standoff, but allowed the teacher to speak with authorities by phone.
During the standoff, dozens of people kept away from the scene by firefighters congregated in the parking lot of a nearby hair salon in the town of about 12,000 people that sits on the border with Michigan's Upper Peninsula.
"This is not a good mood, there are a lot of people who are scared, a lot of students who are worried," said Jason Chaltry, 32, whose nephew is a sophomore at the school but was not in the classroom.
The high school has an annual enrollment of approximately 800 students, according to its website.
"They just spent a whole bundle of money on classroom doors to make them secure, but they don't have metal detectors at the school," said councilman Bradley Behrendt.
Choral teacher Bonita Weydt said she was talking with a teacher in another classroom after school, which lets out about 3:10 p.m., when principal Corry Lambie came in.
"I said, `Corry, what's going on?' and he said, `Get out of the building,"' Weydt said.
Officials had asked parents to gather at the county courthouse, where school officials met with families.
Marinette Schools Superintendent Tim Baneck said the community had gone through an emergency response training exercise last year.
"So the local law enforcement officials as well as the educators were all involved in a mock shooter situation, so it is actually very fresh in our minds in terms of the training we just went through," he said.