Shon Barnes, police chief in the state capital Madison, told a news conference that a teacher and a teenage student died at the Abundant Life Christian School, a private Christian school for children aged five to 18.
He declined to share the gender or exact age of the suspect, who attended the school of around 400 pupils, saying "as difficult as today is, that's still someone's child that's gone."
Two students were being treated for life-threatening injuries, while four others were in hospital in a non-critical condition.
A handgun was recovered at the scene, he said, adding that the suspect's family was cooperating with the police investigation.
Monday's violent episode is the latest in a long line of school shootings in the United States, where guns outnumber people and attempts to restrict access to firearms face perennial political deadlock.
Underlining the commonplace nature of mass shootings, Barnes said that some medical personnel responding to Abundant Life came directly from training for such an event.
"I think we can all agree that enough is enough," Barnes told reporters.
"We have to come together to do everything we can to support our students, to prevent press conferences like these from happening again and again and again."
Horror of school shootings
Police were alerted to the shooting by someone at the school shortly before 11:00 am (1700 GMT).
"When officers arrived, they found multiple victims suffering from gunshot wounds" and also "located a juvenile who they believe was responsible for this, deceased in the building," Barnes said.
"We believe the shooter was a student at the school," he added, saying police officers "never fired their weapons."
The White House said that President Joe Biden, who has fought largely unsuccessfully for tighter gun laws, had been briefed on the shooting.
Wisconsin Governor Tony Evers said in a statement that "we are praying for the kids, educators, and entire Abundant Life school community as we await more information."
This year, there have been at least 487 mass shootings -- defined as a shooting involving at least four victims, dead or wounded -- across the United States, according to the Gun Violence Archive.
At least 15,998 people have been killed in firearms violence in the United States this year, according to the GVA.
In early September, a 14-year-old boy killed four people, including two students, at a high school in the state of Georgia, before being taken into custody.
Nineteen students and two teachers were shot dead in May 2022 when an 18-year-old gunman stormed their Uvalde, Texas elementary school and opened fire.
