The defrocked priest, 81-year-old August M., was sentenced to 12 years in prison in 2013 for 24 cases of child abuse and sexual violence at the school in Upper Austria.
He was charged with sexually abusing 15 children whilst head of the school. He was also accused of torturing and neglecting young boys under his care between 1973 and 1993.
A search at the monastery in 2010 revealed that August M. owned a pump-action shotgun amongst other weapons.
He is said to have whipped, kicked and slapped his students, and to have occasionally threatened to go and get his shotgun.
August M. admitted that he was guilty but asked for an acquittal under the statute of limitations - which was rejected.
The regional high court in Linz must now decide the total length of his sentence and also if he should pay damages to any of his victims.
One of his victims told the Austrian Press Agency that the court’s decision was a relief. “It gives one the feeling that you can rely on justice to be done. It’s a good sign - but just a drop in the ocean,” he said.
A lawyer for the victims, Johannes Öhlböck, called the judgement a “milestone for a lawsuit of this size and nature”.
Abuse scandals in Austrian Catholic institutions have badly damaged the religion’s image.
Following countless allegations of sexual abuse a commission was set up by Vienna's archbishop, Cardinal Christoph Schönborn, to provide an independent, secular mandate for investigating claims.
It dedicated €8 million from a compensation fund set up by the Catholic Church for compensating victims in cases found to be merited.