Boys' English grades are up to a tenth worse when high numbers of girls are in the class with them, though girls' grades are unaffected.
Boys do worse in English when there are girls in their class, researchers will say today, contradicting the widely held belief that girls are always a good influence on boys in school.
Boys do best with "as few girls as possible" in English lessons at primary and secondary school, Steven Proud, a research student at Bristol University, will tell the Royal Economic Society's conference.
But when it comes to maths and science, both boys and girls at primary school achieve up to a tenth of a grade more when there is a high proportion of girls in the class, Proud found.
Proud tracked boys' and girls' test results at the ages of seven, 11, 14 and 16 in 16,000 schools in England between 2002 and 2004 for his PhD.
He analysed the test scores to see whether the proportion of girls in a year group made a difference to the results of both genders in maths, science and English.
There are marginally more boys than girls in schools, but most classes in mixed schools are almost equally split between the genders. Proud looked at these and schools that were exceptional in their high or low proportion of girls.
Boys consistently perform up to a tenth of a grade worse when they study English with high numbers of girls as opposed to few or no girls, Proud found.
The more girls there are in an English class, the worse boys perform. This is particularly the case in primary schools, he discovered.
Proud will argue that his results show boys should be taught English in single-sex classes.