When a little boy burst into tears in her third-grade classroom last fall, Audrey Jost pulled him aside while the other kids were at recess. He said he was getting headaches and falling behind the other students, so Jost spoke with his parents about getting his eyes tested. A few days later the boy returned with contact lenses, seeming like a different kid, said Jost, who teaches in a public school in Gilbert, Arizona.

“It was amazing, just the change that that had in him, academically, socially,” she said. “He just lit up that day.” In her 18 years of teaching, Jost said she’s seen this problem — and its relatively simple solution — many times before, including with one of her own daughters.

That’s why experts say late summer is a great moment to add an eye test to the back-to-school checklist. The American Optometric Association says 1 in 4 children start school with some sort of vision problem, impairing their ability to learn, participate in sports and observe the world around them. But many kids won’t tell you something is wrong because they don’t know they have a problem, said Annette Webb, an optometrist in Hot Springs, Arkansas.

“They don’t have anything to compare it to,” said Webb, who recounted a story about a boy who loved to draw trees but had never seen individual leaves until he got glasses. “They think everybody sees like they do.” Many children with undiagnosed vision issues can develop behavior problems because they are predispo.