Maths teachers have complained that there were no questions on trigonometry in the intermediate maths paper set by Edexcel. More than two-thirds of schools use the board's GCSE maths exams.
Doug French, the president of the Maths Association, said: "The exam board might say that it is luck of the draw, but I feel concerned that trigonometry is not being tested as a matter of course."
Jenny Golding, also of the Maths Association, said this year's intermediate and higher papers had both been easier than in previous years. "A number of teachers have told me that their pupils found the exams easy," she said.
"The problem is that an easier paper does not rank pupils in the same way as a harder one. It does not differentiate between pupils in the same way."
Under the intermediate paper, pupils can achieve up to a grade B. It is being phased out next year, leaving a two-tier system of foundation and higher papers.
Anne Hendy, the head of maths at Hitchin girls' school in Hertfordshire, who has been a teacher for 40 years, said it was the first time she could remember there not being a trigonometry question on the intermediate paper.
Her school is planning to complain to Edexcel. "We were absolutely incensed," she said. "Trigonometry calls for problem solving skills that are not covered in other parts of the syllabus."
A spokeswoman for Edexcel said it was not possible to test all parts of the syllabus.