David Grigg, head of Lord Lawson of Beamish secondary, County Durham, said that the language used in questions was more sophisticated than that used in some GCSEs, where a reading age of only 11 is required.
He said: "The Qualifications and Curriculum Authority has to look into this. It looks as if pupils need to be at level 6 or 7 for reading just to know what is being asked of them at key stage 3 science.
"The test may be more an assessment of reading than of science."
Mr Grigg, a former head of English, found that in two one-hour KS3 papers the questions were made up from a total of 5,652 words. By contrast, two 90-minute GCSE foundation double-award papers, set by AQA, had only 3,592 words. He then used the Fry test, developed in the 1960s, to calculate the reading ages needed for the papers.
Questions seen by The TES showed substantial differences in style. The KS3 papers featured lengthy preambles, while most of the GCSE problems were asked without an introduction.
A spokesperson for the National Assessment Authority, part of the QCA, said: "In 2003, new questions were introduced ... which do require more context than other questions. This type of question will increasingly be used in GCSE too. This change has been widely welcomed by science teachers."