He said the school would be working closely with MHS and police to ensure all students involved were identified.
"Any student who has engaged in violence will face serious disciplinary action within the school and will also be reported to police," McMillan said
The large group of young people were gathered at the intersection of Weymouth Rd and Rowandale Ave.
Police dispersed the group and no charges were laid, a police spokeswoman said. She was not aware of any injuries and no ambulances appeared to have been called.
McMillan was pleading for families to talk with their children and remind them that violence has no place at school or the community.
"I also encourage our families and whanau to advise us of anything that they know so we can do our best to ensure that this is dealt with fully and does not occur again," McMillan said.
"The many hundreds of hardworking and praiseworthy students who attend our schools, their families and whanau do not deserve to have their pride in their schools and our community tainted in this way.
"Our staff and I will leave no stone unturned to address this."
Manurewa High School principal Pete Jones yesterday told the Herald it had a very positive relationship with both James Cook and the police and would work with them to identify any of its students involved.
The school had a strong no-violence policy and promoted its "Manurewa values" of respect, excellence, whanaungatanga [kinship] and akoranga [learning/teaching], Jones said.
"Any student who has been involved in acts of violence will face appropriate disciplinary action at school as well as the appropriate police action.
"I would ask any of our whanau to talk to their rangatahi and share with us any information as well as promoting the message that violence is never OK and has no place in our community."
Social media posts have suggested Manurewa students were lying in wait for James Cook students coming home from school through nearby Mountfort Park, while others said there were weapons involved.
Police have not confirmed those reports.