According to the police, it was an isolated incident, the suspect is already in custody, while the building is safe and secure.
Waukesha South is on lockdown, as is the nearby Whittier Elementary School.
The police reportedly responded to the incident at 10:15 a.m. local time (16:15 GMT). Multiple police cars, ambulances, and the fire department have promptly arrived at the site.
ABC News reported that at least one person is wounded.
There is an active police situation at South High School. One young man was loaded into an ambulance on a stretcher. We will provide more details as they become available.
NBC News, citing a school district official, reported that the incident saw a school security officer exchanged gunfire with a suspected student gunman. The school security officer struck the student during the exchange, and his condition is not immediately known, according to the report.