Nervous teenagers will turn out to pick up their GCSE results today.
Schools that used to play the system by entering pupils for exams early and multiple times could be in for a "sharp shock" when GCSE results are published, it has been suggested.
Those that relied heavily on "gaming" are more likely to see lower grades, according to Professor Alan Smithers.
Scroll down for live updates as Essex pupils pick up their results
His comments come following warnings from England's exams regulator, Ofqual, that there is likely to be "variability" in grades at a school level this summer due to the significant changes to the qualifications.
A move to end-of-course exams, rather than exams throughout, more students taking international GCSEs (IGCSE), cuts to re-sits, a toughening up of GCSE geography, and a decision by Government that only a youngster's first attempt at a GCSE will count in school league tables are all likely to affect this year's results, the regulator said.
Further changes, such as speaking and listening assessments no longer counting towards a student's overall GCSE English grade and a move towards more exams and less coursework in the subject, could also have an impact.
Prof Smithers, director of the Centre for Education and Employment at Buckingham University said: "Individual schools are likely to be affected differently according to how much they relied on gaming the old system so for some there could be sharp shocks in store."
Last year's results showed a shift towards pupils studying traditional academic subjects, with an increased number of entries for the three separate sciences, as well as a reversal in the long-term decline of foreign languages.
It was suggested that the hikes could be down to the introduction of the Government's English Baccalaureate (EBacc), which is awarded to pupils who score at least a C at GCSE in English, maths, science, history or geography, and a foreign language.
But there are indications this year that entries numbers in some subjects are unlikely to rise again this year.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), added: "The constant piecemeal changes to GCSE exams are making it increasingly difficult for schools to prepare for GCSE exams and to accurately predict what students will achieve. Despite constantly shifting exam goal posts, teachers and students have worked incredibly hard to achieve the results that they will receive in the morning.
"If the Government wants to do its part to help young people get the education they need to succeed in life, it must stop this obsession with tinkering with exams."