The interior ministry said there were several fatalities but did not specify how many.
However the local Kronen Zeitung newspaper said eight people have been confirmed dead in the school shooting.
The fatalities included the suspected attacker, Austrian broadcaster ORF quoted the interior ministry as saying.
Several serious injuries included students and teachers, police spokesman Fritz Grundnig told local media.
Special forces were among those sent to the BORG Dreierschützengasse High School after a call at 10am local time. Police said on social network X about an hour later that the deployment was still ongoing and that it involved a helicopter.
"Currently, a police operation is under way ... The reason for the deployment was that gunshots were heard in the building," the police said on X, confirming the attack in Graz.
Graz, Austria's second-biggest city, is located in the southeast of the country and has about 300,000 inhabitants.
Attacks in public are rare in the Alpine nation of almost 9.2 million people.
In February, a stabbing left a teenager dead and five other people injured in southern Austria, with a 23-year-old Syrian asylum seeker held.
MNA/
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