BBC News, at the ASCL conference
Pupils who opt for "easier" subjects such as sport or drama rather than sciences or languages could achieve higher GCSE grades, research suggests.
Ministers have resisted claims some exams are easier than others
A Durham University study concludes some subjects are easier than others - because of the way exams are graded.
Researchers have worked out a scale of subject difficulty using the GCSE results of 650,000 pupils from 2004.
They say a student might get a grade higher if they opted for drama or media studies over chemistry or German.
The findings - by Dr Robert Coe at Durham University - were highlighted just as a report on the teaching of languages in schools is about to be released.
The report, by Sir Ron Dearing, is due on Monday. It is expected to come out in favour of all children being taught languages at primary school.
There is concern about the falling numbers of teenagers opting to do modern foreign languages - which are no longer compulsory in secondary schools after the age of 14.
Last year there was a 14% fall in the number of entries for German at GCSE and a 13% drop in those for French.
For the Durham study, carried out at the university's educational research unit, the CEM Centre, researchers looked at children's academic abilities and their results in various subjects.
Dr Coe told head teachers meeting at a conference in London: "There is more than one grade's difference between Spanish and German against drama, textiles or media studies.
"That's a big difference."
Dr Coe told delegates not to "feel bad" if their subject was seen as easier: "It's not to do with the subject, it's to do with the grading. It reflects the entry that the different subjects have.
"In the 1980s, grades were set to a proportion of the entry, so getting a grade A was harder."
Standards had been maintained, he said.
Carolyn Hann, from the Association of School and College Leaders (ASCL) said: "It's harder to get a C in maths and what are we going to do about it? Put pressure on the grading bodies to help get Cs."
Ministers and England's exams watchdog the QCA have long resisted suggestions that some exams are easier than others.
Grade conversion call
But some universities - such as Cambridge - do not count certain GCSE subjects in their minimum entry requirements because they do not think they show general academic ability.
GCSEs however, do merit equal points on the league tables. Schools are measured on the proportion of children achieving five good GCSEs (grades A* to C). Last year was the first time the five GCSEs had to include maths and English.
Dr Coe said: "We accept that there are differences but if you are in the situation where we are trying to treat them as equal, for example in league tables or on UCAS forms, we should have some kind of conversion to put them on the same scale, to standardise them".
This happened in Australia, he said, where it was accepted that some subjects were harder than others.
For university entrance, there was a scaling process whereby subjects were weighted depending on their difficulty.