While Marinette Police Chief Jeff Skorik told reporters all of the hostages had been freed, the gunman's status wasn't clear.
The Green Bay Press-Gazette reported the Marinette County Sheriff's Department said the 5-hour standoff between the armed student and authorities had ended.
WFRV-TV, Green Bay, reported the suspect had been armed with a handgun and police had contact through a teacher who had been taken hostage along with 23 students. The gunman had not made any demands and his motive wasn't known, the TV station said.
No one was injured, WFRV said.
The Press-Gazette reported the gunman initially released five hostages and then later let the remaining 18 students go. The teacher remained with the gunman for a time, the newspaper said.
Police had "no reason to believe there is any more than one actor," Skorik said.
Marinette is 50 miles north of Green Bay and has a population of about 11,700.