At Flaxmere Primary School in Hawke's Bay seeing is believing.
Optometrists visited the decile one school to see how many kids have hidden vision problems.
"That has a flow on effect to your school life, your home life, your social interaction, your sports," one of the optometrists told Seven Sharp.
However, the results from the students' tests were a worry.
"We've got 30 to 40 per cent of our kids who aren't seeing as well as they could be, and therefore not achieving as well as they could be," the school's principal Robyn Isaacson said.
While, one student said: "One time we were doing handwriting and I couldn't see the letters properly and I got in trouble."
The kids will have future vision testing for free.