Jack said the 17-year-old suspect kept his hands in his pockets while officers tried to de-escalate the situation.
"The suspect removed the handgun from his waistband and pointed it at the officers," Jack said. "An officer was forced to discharge his firearm, striking the suspect."
Jack said the officer who shot the suspect has been with the police department for 11 years.
Students told the Milwaukee Journal Sentinel that they heard at least two gunshots about 20 minutes after the lockdown began.
"They were right next to my classroom, so we all went into shock," a student, who was not identified by name, told the Journal Sentinel. "Even my teacher hid with us. We stayed down for like, 20 minutes, and the actual lockdown sound went out."
Jack said investigators recovered a firearm from the scene. No other people were believed to have been involved in the incident.
"This is clearly a superintendent's worst nightmare," Waukesha Schools Superintendent Todd Gray said. "We are very grateful for the response of the South High School SRO, the Waukesha Police Department and the Waukesha County Sheriff's Department."
Police continue to investigate.