The Home Office said there were several deaths, but did not identify how many.
However, the local Kronen Zeitung newspaper said eight people were confirmed dead in the school shooting.
The deaths included a suspicious attacker, and Austrian broadcaster ORF cited the Ministry of Home Affairs saying it.
Police spokesman Fritz Gundnig told local media that some serious injuries included students and teachers.
The special forces were among those sent to Borg Dreierschzengas High School after calling at 10am local time. Police said on Social Network X about an hour later that the deployment was still ongoing and that helicopters were involved.
“A police operation is currently underway… The reason for the deployment is because gunshots were heard in the building,” police confirmed the Glaz attack on X.
Glaz, Austria’s second largest city, is located in the southeastern part of the country and has around 300,000 residents.
Public attacks are rare in alpine countries with almost 9.2 million.
In February, a stab wound killed a teenager and five others injured in southern Austria, hosting a 23-year-old Syrian asylum seeker.
MNA/