Citing local police, Austrian state media including national broadcaster ORF said several people had been seriously injured, including students and teachers.
The Interior Ministry said there were several fatalities but did not specify how many.
Special forces were among those sent to the BORG Dreierschützengasse high school after a call at 10am.
At 11.30am, police wrote on social network X that the school had been evacuated and everyone had been taken to a safe meeting point.
It was not immediately clear whether the suspect was among the victims.
Kronen Zeitung said a suspect had reportedly been found dead in a bathroom, but the report could not immediately be confirmed. (Reuters/AP)