The woman, who spoke on the condition of anonymity, told Rotorua's Daily Post newspaper that her grandson was among the five, who were stood down for three days.
She said one of the other boys had brought the joint to school.
She feared that if children as young as her grandson were smoking drugs at Owhata, then it could be happening elsewhere.
"It was terrible," she said. "I'm very concerned. I was shocked it was at a primary school."
The woman said she had spoken to her grandson about what happened but she did not think he realised the severity of his actions.
She thought the school could have used other penalties rather than standing down her grandson, arguing he would have enjoyed three days off school.
"Maybe clean up the school grounds or that type of thing, something he doesn't like to do."
Owhata principal Bob Stiles said it was the first time drugs had been found on the school premises this year.
"It is something that does happen and it happens in every school."
He said the incident was dealt with promptly and efficiently.
"All the parents came in and were very supportive."
Stiles was positive the drugs did not come from any of the pupils' homes and said the boy who brought the joint said he had found it on the way to school.
All five were given drug education and the whole school was warned about the dangers of drugs.
Rotorua Principals Association president Colin Watkins said cases of drugs being found in primary schools were extremely rare.
"Even the instances of cigarettes coming into school is practically zero."
Auckland Primary Principals Association president Owen Alexander also said the case was rare.
"It's one out of the box. I'm sure most principals would be disappointed if this were a new trend.
"If 9-year-olds are bringing it to school, what sort of environment are they bringing it from?
"How are they getting hold of it?" asked Alexander, who is also principal of Takapuna Normal Intermediate School.
Figures previously released to the Herald on Sunday showed that in 2004, at least 3116 students were stood down or suspended for bringing drugs and alcohol to school. Up to 135 were aged between 5 and 10.
One was a 10-year-old boy stood down from Kerikeri Primary after handing out cannabis in the playground.
The school's principal said the boy got it from his older brother and brought it to school to show off.
Drug abuse was the third most common cause of student suspensions last year (20.1 per cent), behind continual disobedience (27.5 per cent) and physical assault on other students or staff (23.8 per cent).
- ADDITIONAL REPORTING BY ROTORUA DAILY POST
