A eyesight screening test at Flaxmere Primary School found that 42 per cent of the students had conditions requiring further examination.
The international charity Essilor Vision Foundation looked for hidden vision problems in Years 4-7 students, taking two days with specialist equipment and volunteer optometrists.
Grant & Douglas optometrist Jenny Stewart said many students referred would not need glasses.
It was "just a screening test" and many conditions were minor, but the number referred was higher than the expected 30 per cent.
She was concerned that higher-than-average referrals might be indicative of a widespread problem. "If these findings are extrapolated out through the more than 227,000 children aged 9-12 in the rest of country, it means there are potentially thousands of Kiwi children whose learning may be impaired due to eyesight issues."
Children beginning school were screened but their eyes were not mature enough to detect additional conditions.
Children with vision problems could easily be labelled as lazy or difficult. The study found a large number of cases where eyes did not track well, making reading difficult.
"This is a real shame as often it can be down to eyesight issues which haven't been picked up, as their eyes can compensate for the vision issue to some degree, but the child can have some underlying discomfort which can put them off reading."
School principal Robyn Isaacson was surprised at the results. Parents had been told and further free testing was available. The school would compare data with achievement levels to see if glasses made a difference.
Thankfully, wearing glasses held no social stigma. "Our kids will often walk around with empty frames because it's part of a look," Ms Isaacson said.
Mrs Stewart said if children had headaches, complained about reading, rubbed their eyes, covered one eye when reading or tilted their head to see things more clearly it was important to get them checked by a specialist.
Screening at optometrists is subsidised for Community Services Card holders under 16.