The condition of the gunman wasn't immediately known, and police have yet to identify a motive behind the standoff, which rattled the community of Marinette, 50 miles north of Green Bay.
Police had been in communication with the female teacher inside the classroom, who confirmed to them that she and the students were being held hostage at gunpoint, Police Chief Jeff Skorik said. Five students were released Monday evening. A short time later, the rest of the hostages were let go.
A school administrator reported the hostage standoff at the end of the school day, 3:48 p.m. local time Monday, saying the student burst into the classroom and took hostages.
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.
Parents were asked to respond to the county courthouse to receive information on their children.
A SWAT team had arrived at the school Monday evening, city councilman Bradley Behrendt said from the scene about 50 miles north of Green Bay.
"I would say there's over 100 officers here, everyone from Marinette County, Green Bay ... It's very shocking," Behrendt said. "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."
Marinette sits on the border with Michigan's Upper Peninsula. The high school has an annual enrollment of approximately 800 students, according to its website.