Several students and at least one adult, as well as the suspected gunman, were among the dead, Mayor Elke Kerr confirmed to the Austrian news agency APA.
School shootings are much rarer in Europe than in the United States, but in recent years Europe has been rocked by attacks on schools and universities that were not linked to terrorism.
"A police operation is currently underway... The reason for its deployment is that shots were heard inside the building," police wrote in X, confirming the attack in the city of Graz.
"The situation is very unclear at the moment," police sources were quoted by AFP as saying.
The EU's top diplomat, Kaja Kalas, said she was "deeply shocked" by reports of the shooting.
"Every child should feel safe at school and be able to learn without fear or violence," Kalas wrote on X.
"My thoughts are with the victims, their families and the Austrian people at this dark time," she added.
Attacks in public places are rare in the Alpine country with a population of almost 9.2 million. It ranks among the ten safest countries in the world according to the Global Peace Index. | BGNES