He then walked out, followed by the teacher asking him to come back in.
"He came in, ran out the other door, hitting children that he walked past," the teacher said.
"I asked him to go to office, he ran down boardwalk, hitting children as he went, kicked another male child in penis area, climbed a pole. When a child tried to pull him down he punched him, then ran back towards the classroom, pushed a child who fell and hit his head against the pole."
He ran around the playground, pulled a boy off the swing and punched and kicked him to the ground. Then he ran into the hall and locked everyone out.
He finally calmed down later in the playground. The school said six children were hurt.
It wasn't the first time. He was suspended for six days last December after a violent incident during a soccer game. Thompson said he was often called by the school to take Sterling home to calm down after other incidents.
St Columba's principal Gareth Duncan said he employed a trained teacher last year costing $74,460 a year to work with four children with behaviour problems, spending most of her time with Sterling.
However, this year he needed the teaching role in regular classrooms and reverted to using teacher aides.
"We have done what we could with open hearts," he said.
"When schools are faced with high-end disruptive and aggressive behaviours [we have to consider] the safety of, in this case, my 500 other children."
After excluding Sterling, Duncan asked three nearby schools to take him: Aberdeen, Frankton and Rhode St.
Rhode St principal Shane Ngatai said his school often took in students expelled from other schools, but he had reached his limit with several high-needs autistic students already in Years 3 and 4 classes taught by beginning teachers.
"I have had to stand up on behalf of my community and my staff because they are getting burnt out," he said.
Aberdeen and Frankton both have school zones and said they were already turning away other children who lived outside their zones.
The Thompsons moved from Hamilton to Ngaruawahia last year, but want to keep Sterling in one of the six schools that have a bus service to the Kidventure after-school service, which Thompson said provided "a stable environment" for Sterling. Sterling's older brother Rohan is still at St Columba's and also attends Kidventure.
Thompson also wants a school close to his work so that he can get there quickly when necessary.
The Education Ministry's head of sector enablement and support Katrina Casey said the family had rejected three schools, including one on the Kidventure bus route (Nawton), along with Rhode St and Crawshaw.
The ministry and the family are now arranging an interview at another school on the bus route, Forest Lake.
"We're surprised that the family has apparently lost patience after two and a half weeks while we are still actively working through options with them," Casey said.
She said the ministry's Severe Behaviour Service helped to develop a plan for Sterling at St Columba's.
"The behaviour plan for Sterling had worked at St Columba's until the most recent incident," she said.
"We will put in place appropriate support to ensure Sterling makes a good transition to his new school."
Autism NZ chief executive Dane Dougan said too many autistic children faced problems getting into schools.
"We are seeing more and more of these types of situations. In my three and a half years here I think it has increased dramatically."
He said 1.5 per cent of New Zealand children were on the autistic spectrum, but funding for teacher aides and other support under the Ongoing Resourcing Scheme (ORS) was limited to about 1 per cent of students.
The ministry said a number of options for additional support were available for students with autism, depending on their level of need.
The ORS scheme was only for the most severely needy children. The Severe Behaviour Service and speech language therapy were available for the next most needy 2 per cent of children.
