"The boys had a competition where they would get young girls drunk and they would dangle their genitalia over their faces and take photos," he said.
"The competition was how many girls you could get into those compromising photos."
He advised the school to let the police handle any inquiry.
"The police were involved. The boys involved received a warning and weren't prosecuted," said Walsh.
"What they are doing is criminal and totally unacceptable. In my view they do need to be charged, convicted and a message [sent] to teenagers across the country that this is totally unacceptable."
Police would not comment on the case but said: "We take all allegations of sexual misconduct and assault very seriously and investigate them appropriately."
The incident has echoes of the infamous Roast Busters scandal two years ago in which a group of young men claimed to have got young girls drunk before having sex with them. They bragged about their alleged exploits on a Facebook page.
No charges were laid, despite several official complaints, including from a 13-year-old girl.
An Independent Police Conduct Authority of New Zealand report released in March criticised the initial police investigation, saying it had let down alleged victims.
National sexual violence survivor advocate Louise Nicholas said it was concerning the teenagers in the latest cases weren't held to account. "For boys of that age, with that type of behaviour, a warning is a joke."
Instead, schools needed to bring in agencies dealing with sexual-harm behaviour and educate the teens involved to understand the harm and to avert criminal activity.
It was disappointing many schools and parents remained in denial about the extent of this type of abuse and refused instructive programmes in classrooms.
She called for the Ministry of Education to make courses addressing Roast Buster-type behaviour mandatory in schools.
Walsh said the the cases backed disturbing findings from an earlier survey that showed the Roast Busters case was not an isolated incident.
He said the time had come to stop a destructive teen culture, adding there was a hardcore group of schoolboys who thought they were entitled to harass, bully and intimidate others using technology.
"I don't think they should go to prison. [Instead] fines, community work and compulsory attendance at programmes to address their attitude should be part of the penalty."
Auckland University law lecturer Dr Bill Hodge said there was scope under the new Harmful Digital Communications Act to put the statute to the test in incidents like those.
"This seems worse than bullying but it would seem to fit into that and would be something worth exploring," said Hodge.
