A lack of diligence and attention at school could be early signs of illness
Poor performance at school could indicate an increased risk of later developing schizophrenia, a study says.
UK and Swedish researchers followed more than 900,000 children born between 1973 and 1983.
The Psychological Medicine paper found getting an E grade in any GCSE-stage exam was linked to a doubling of the small risk of developing schizophrenia.
But a mental health charity said the illness was often linked with high, rather than low, intelligence.
Schizophrenia, which commonly causes people to hear voices and experience paranoid delusions, often becomes evident in the late teens or early 20s.
The researchers, from the Institute of Psychiatry at King's College London, and the Karolinska Institute in Stockholm, looked at Swedish data on exam results taken at the age of 15 or 16.
They then looked at hospital data on admissions for psychotic disorders including schizophrenia after the age of 17.
Sweden has comprehensive national registers, with every individual having their own identification code, so the data could be compared.
The general risk for an adult to be diagnosed with schizophrenia in any given year is seven in 100,000.
Getting an E grade in any of the 16 subjects looked at by the researchers was linked to a doubling of that risk.
The researchers found those with the poorest school performance overall had four times that risk of developing schizophrenia when they were adults.
Other studies have shown that there is a link between schizophrenia and earlier problems with learning or understanding.
However, the researchers said other factors were probably involved.
Writing in the journal, Dr James MacCabe of the Institute of Psychiatry, who led the research team, said: "School performance should not be seen simply as a proxy for intelligence."
The researchers said poor attendance and engagement with education, memory and attention problems as well as issues with organisation, creativity, diligence and social skills could all play a part.
Dr MacCabe said: "Doing badly at school is not a cause of schizophrenia, but it is a marker for something not being quite right several years prior to diagnosis.
"This isn't going to be a way of identifying people at school who are at risk of developing schizophrenia.
"But it could be useful when considering someone who is displaying other potential symptoms of impending psychosis."
But Hilary Caprani, a spokeswoman for the mental health charity Rethink, warned: "It is important to recognise that mental illnesses like schizophrenia are not linked to low intelligence. The opposite is often true.
"There are lots of reasons why young people perform poorly in exams.
"Problems with concentration and mental distress can interfere with studying and these can also be early signs of mental illness - symptoms that commonly begin in late teens.
"The people in the study might have had normal intelligence but started having low-level symptoms that disrupted their schooling."
She added: "The good news is that many people who have psychosis recover and go on to have challenging careers."