Dithebe, who was also in detention, told the commission he was beaten by police when he intervened, SABC radio news reported.
The victim, Sipho Mutsi, a Standard Four pupil, died as a result of the assault. Mutsi was a member of the Congress of South African Students.
Dithebe said police influenced some of Mutsi's teachers at his school to testify at the inquest that he was epileptic and had died of a brain haemorrhage.
Sipho's mother, Irene Mutsi, told the commission she was not allowed to attend the post moterm on the death of her son.