The court heard Robertson’s victims had been sent to the residential school for “short-term respite” from their homes.
Robertson, now of Essex, admitted she used a “strong voice” when sometimes dealing with the girls, but denied being violent and bullying them.
The trial heard that the teacher force-fed, assaulted and humiliated the girls.
Fornethy was run by two former local authorities, Glasgow Corporation and Strathclyde Regional Council.
It closed in 1993 and the building has been empty ever since.
It was said to offer “convalescent care” for girls aged between five and 12 for up to eight weeks at a time.
A number of former residents gave evidence about the regime there at the trial.
One of the victims recalled being force-fed semolina, which she had been struggling to swallow.
The woman said she was then made to stand in a corner of the room facing away from others.
She said this sort of incident would “happen to everyone”.
The victim also said she was assaulted during a later stay at Fornethy while helping to scrub floors.
She was pulled back and forward “like a rag doll” before ending up on the floor on her knees.