When he returned to the sideline, he was hit in the head by the Kelston parent, who called him a cheat.
Evans said the man was very remorseful.
"The punishment of not being able to watch your kid play sport is pretty tough. He understands the severity of it."
Evans said the ban was for at least the next year but would be reviewed.
He said Kohlhase had made it clear that he did not want the police involved, and the school accepted that. "We would have supported [a police complaint] if that was what he wanted."
St Peter's principal Kieran Fouhy said he trusted that Kelston had dealt with the incident appropriately. "They are a good school, they will do the right thing."