The former Parktown Boys High water polo coach has been found guilty on 144 charges of sexual assault, as well as 12 charges of common assault.
The 22-year-old was initially facing more than 300 charges including rape, exposing minors to pornographic material, relating to the sexual abuse of minors at the school.
In September, Collen Rex pleaded guilty to 144 of the charges.
The abuse came to light after the accused was implicated by footage from a surveillance camera at the school’s hostel in November 2016.
Rex has only been found guilty on 156 charges.
At the start of the case, Rex pleaded guilty to 144 of those charges, saying he was young and immature at the time and he regrets his actions.
In his judgment, Judge Peet Johnson said that the state failed to prove its case beyond reasonable doubt in regards to the charges of rape, exposing minors to pornographic material as well as attempted murder.