A Northland College student was allegedly held down by youths who entered the school grounds before being stabbed in one ear, while trying to defend a teacher.

The source, who spoke to the Northland Age on condition of anonymity, and did not disclose the gender of the student, said the stabbed teen collapsed and had to be revived before being taken to Bay of Islands Hospital on Friday.

The injury was not believed to be life-threatening.

The Northland Age understands there were seven intruders, aged 14 to 18 years, none of whom attend the school, although reports on Friday indicated that there were three offenders.

According to a witness there had been an incident in Kaikohe the day before, where a teacher had stepped in to prevent a student from being hurt. That evening the teacher was set upon and beaten to the ground.

At about 11am on Friday the youths entered a classroom. One of them, who was armed with a knife, reportedly threatened to stab the teacher who had intervened earlier, and to assault a student. Students stepped in to form a barrier between the teacher and the teenagers while other teaching staff rushed to help.

Three other staff members were then set upon, one, who suffered cracked ribs and other injuries, reportedly resigning a short time later.

According to reports on Friday it was one teacher and two students who were attacked by one teenager. One of the students had been in a moderate to serious condition, after collapsing, when emergency services arrived.

The big question, according to a Facebook post, was what would be done to improve security, to protect teachers and students from a similar incident in the future.

Principal John Kendal posted a statement on the school's Facebook page on Friday: "You may be aware that a serious incident occurred at school today. Several youths (who are not students at Northland College) came on to our grounds and assaulted some staff and students.

"We are working closely with the police, and they are currently investigating this situation."

Several parents responded by expressing concerns about security at the college, but Mr Kendal said the school believed it was an isolated incident.

"As a school we will liaise with the police and reflect on this further, so that we make sure our safety procedures are as strong as possible," he added.

"We, along with the Ministry of Education, will continue to support our students and staff as needed, and ensure that the school is settled so that there is no disruption to teaching and learning programmes next week."

Police confirmed that a 17-year-old, who was living with relatives in Kaikohe, was helping with their inquiries.