Fears about wild swings in students’ grades are growing ahead of GCSE results day tomorrow, with English and maths causing particular concern among school leaders.

Several teachers have contacted TES to express surprise at their students’ results. Brian Lightman, general secretary of the Association of School and College Leaders, said a number of the union’s members had come forward with concerns about “surprises” in their grades.

“Clearly some schools have had some surprises, which they certainly didn’t want,” Mr Lightman said. “English is the main one, and we’re also hearing about some surprises in maths.”

One head, who asked not to be named, reported a drop of 18 percentage points in the proportion of students achieving at least a grade C in English, which she said was due to the grade boundaries moving upwards.

“We’re absolutely furious,” she said. “Our overall results are now below the floor target [of 50 per cent being introduced next year], and it’s all down to English. The kids are going to be distraught.”

Another teacher reported a “significant drop in English language A*-C pass rate”.

In maths, a teacher told TES that the majority of his school’s students had achieved “at least one grade lower” than predicted, while another said the pass rate had “dipped by 5 per cent”.

Professor Alan Smithers, director of the Centre for Education and Employment at Buckingham University, said schools that in previous years had relied on early and multiple exam entries could be in for a “sharp shock”.

Exams regulator Ofqual has warned that there is likely to be “variability” in grades at a school level this summer due to the significant changes to the qualifications. These include cuts to resits, a toughening up of GCSE geography, and a decision by government that only a student’s first attempt at a GCSE will count in school league tables .

In English, speaking and listening assessments no longer count towards a student’s overall GCSE grade, while a higher proportion of overall marks will come from exams rather than assessments.

Sam Freedman, director for impact and research at Teach First, tweeted that maths and English grade boundaries were “very tough” this year, adding that “it will just hit schools with lots on the C/D borderline again”.

In response, Sir David Carter, chief executive of the Cabot Learning Federation, said “maths grade boundaries [are] more of a concern this year from what I am seeing”.

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Ofqual chief warns of exams rollercoaster - August 2014

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Schools face five solid years of constant exam revolution - April 2014

GCSEs: ‘Pass’ grades to get tougher and new recognition for ‘exceptional’ pupils - April 2014