Police were called and a Boys' High student was taken away for questioning.
Mr Walsh said another group of Boys' High students came back to his school during the lunch break but quickly dispersed.
The victim of the alleged assault was shaken, but not seriously hurt and should be back at school today.
Mr Walsh blamed the fight on students making inappropriate comments on Facebook
"There's been what might be described as some inappropriate exchanges on Facebook ... students need to be aware about the comments they put out in cyberspace and that they can lead to some friction ... Facebook has a lot to answer for."
At an assembly after the attack, Mr Walsh issued a "general warning" to his students to walk home in groups and make sure they had a cell phone handy in case they ran into trouble.
"At times like this, when there is tension between the schools, it's prudent to keep some distance until it's sorted out."
He was working closely with Boys' High following the incident.
"One of the boys involved has come back to us to apologise, which was very good of him ... we accepted his apology.
"We have a very good working relationship with Boys' High - we are only talking about a minority here," Mr Walsh said.
Rotorua Boys' High School principal Chris Grinter could not be reached for comment.
A concerned parent, whose child goes to John Paul College, told The Daily Post the incident was not a one-off.
He had told his child not to walk past Boys' High or groups of Boys' High students for fear of bullying or assault because of the uniform his child was wearing.
Rotorua police Inspector Ed Van Den Broek said one child was taken to the police station for questioning and police were deciding whether to lay charges.