A spectator contacted the Herald on Sunday to report the incident because of the paper's Sideline Champs campaign, which is working to improve sideline behaviour.
"It was disgusting," he said.
The incident apparently happened when a linesman couldn't tell whether a ball had gone dead. Kohlhase stepped in and told the referee that it had gone out.
When he returned to the sideline, he was hit from behind by a Kelston supporter, who called him a cheat.
Onlookers said the incident was serious but Kohlhase worked to calm the situation, fearing that it might escalate into something worse.
In 2009, a brawl broke out between Kelston Boys High's first XV and Auckland Grammar, with about 100 students and members of the crowd involved.
It is understood Kohlhase had agreed that he would not report the incident to the police. Instead, he had asked the schools to take action on it.
Kohlhase did not want to comment when approached this week but said he supported the Herald on Sunday campaign and it was important to do something about sideline behaviour.
"I really applaud the initiative," he said.
Auckland Rugby Union board director John Hunt said poor sideline behaviour was "really starting to hurt" rugby's image. "We're trying to stamp it out."
He said he had written a letter to all the clubs in Auckland, expressing his concern at abuse of referees.
"I have asked clubs to take a hardline on it as well. We have zero tolerance for it."
He said the Kohlhase family had a history of connection with the local rugby scene.
"His father was a legend of coaching at Marist."
Club rugby manager Matt McHardy said he had not heard about the incident, but schools usually handled spectator issues. "Our judicial system usually deals only with incidents on the field."
Kohlhase, who coached the Black Sox to a world title this year, is about to take up a new role with High Performance Sport New Zealand.
St Peters principal Kieran Fouhy would not return calls. Someone in the school's sport department said the matter had been dealt with and they were not able to comment.
"It's all in hand. We're pretty firm that we don't want anything reported. It's been sorted."
The director of sport at Kelston Boys High School, Brent Semmons, said he knew nothing about the incident, and Kelston principal Brian Evans did not return calls.