Up to two million children may be falling behind at school because of sight problems, a survey suggests.
The RNIB recommends universal eye-screening for children
The Royal National Institute for the Blind (RNIB) found between 17% and 22% of pupils had an undiscovered eye problem.
Many parents also wrongly believed that schools organised optical tests for children, it said.
The RNIB urged the government to introduce universal screening for
children at ages five, seven and 11.
'Could go blind'
Anita Lightstone, head of eye health at the charity, said: "Children rarely tell their parents that they can't see, as they don't realise they have a sight problem.
"But a free eye test at your local optician can help diagnose a problem.
"Some common conditions such a short and long sight, eye muscle co-ordination problems and most lazy eyes can be corrected and glasses are not always necessary."
NHS eye tests are free for under-16s and for anyone up to the age of 19 who is in full-time education.
In some cases, these can also highlight more serious problems such as cancer, the charity said.
If parents left it too late, their children could even go blind.
The RNIB's study was carried out by Dr David Thomson of City University in London. It looked at 3,000 children in London and the south East between 1997 and 2001.