Aryan was strangled when his clothing became caught on a window latch handle as he tried to climb out of the bathroom near his classroom in May.
He became stuck in the toilet when he was unable to open the door which was left without a door handle due to repairs.
The nine-year-old died in hospital in September more than three months later.
Mr Banerjee said he was told about an incident that happened just months before Aryan's accident where another student was forced to crawl under the toilet door to get out. The couple said they felt there were many ways this could have been avoided and there were still questions that needed asking.
The family took Aryan back to his ancestral homeland and sprinkled his ashes in the Ganges River.
WorkSafe's chief inspector, Keith Stewart, said Aryan's death was a terrible tragedy for his family, the school and the wider Taradale community.
"Your heart cannot help but go out to the Banerjee family. By all accounts Aryan was a bright and bubbly boy and his loss in such a senseless way is unbearably sad," Mr Stewart said. WorkSafe's inquiries did not find any similar incidents involving a window latch handle in New Zealand.
Mr Stewart said although it was possible to foresee that someone might have found themselves stuck in the toilet, "it was not reasonable to think the school or the worker involved should have planned to avoid the possibility of someone being so seriously injured climbing out a ground floor window and being caught on the latch".
"I know the investigation process has been tough for the Banerjees but it was important to take the time to look into this matter and the legal issues surrounding it thoroughly," Mr Stewart said.
WorkSafe informed Aryan's family on Friday about the outcome. The school was also notified. Katrina Casey, head of sector enablement and support, said the Ministry of Education had been in constant contact with the school, wider family and friends since Aryan was fatally injured.
"We have been in touch with them again since the WorkSafe announcement to offer any further support they may require," she said. "This has been a very difficult time for the Banerjee family and we would once again like to express our deepest condolences to Aryan's parents and brother."
Principal Marty Hantz had previously said he was unable to comment while the investigation was still under way. Taradale Primary School Board of Trustees chairman Steve Alexander said, in the only statement that will be released to the public, that the school had always accepted the need for a thorough investigation and it co-operated fully with WorkSafe, "but our focus remains on those who have been devastated by the tragedy".
Mr Alexander said the school and community continued to grieve for the Banerjee family. "We will continue to be in touch with them to offer any support possible."
Aryan's death will now be referred to the coroner.
