Fewer teenagers are being entered for their GCSEs early, according to figures published by England's exams watchdog.
The statistics show a 40% drop in the numbers of youngsters taking the exams before they are 16 (Year 11) - the age at which pupils usually sit GCSEs.
Ofqual suggested that the large fall was down to changes to school league tables - under government reforms only a student's first result in a subject now counts towards a school's results.
The new data showed there had been 505,000 entries from students in Year 10 or below for this summer's exams, down from 842,000 last summer - down 40% overall.
In maths alone, early entries had dropped by 82%, while in English it was down 86% and English language was down 81%.
Ofqual's report said: "The decrease in early entry across all subjects, excluding English literature, is possibly explained by the change to school performance measures announced by the Department for Education in September 2013, when the Secretary of State announced that only a student's first result in a subject would count towards performance tables."
The changes to school league tables came amid growing concerns that schools were entering pupils for GCSEs early, or multiple times, in order to secure good grades.
Tens of thousands of pupils took papers for more than one maths GCSE in 2012 , according to Ofqual figures, while the numbers sitting key English and maths exams before they were 16 also soared.
The importance of gaining at least a C grade in these GCSEs as well as the pressure of league tables on schools was fuelling the move, Ofqual suggested.
Chief regulator Glenys Stacey has previously suggested that multiple entries were being used as a ''tactic'' to help students gain a C grade, and that there was a ''fine balance'' between doing the best for a pupil and demotivating them.
Last summer (2013), 23% of maths entries and 10% of English entries were from students who were not yet at the end of Year 11.
Overall, entries from 15-year-olds increased by 39% from 2012 to 2013.
Education Secretary Michael Gove has previously described early entry as a "damaging trend that is harming the interests of many pupils".
The figures show that English literature has bucked the trend with early entries up 134%.
Ofqual's report also says there are fewer Year 11 entries for GCSEs in English and English language this summer.
This could be because more than 150,000 students took GCSEs in these subjects last November and because schools are increasingly switching from GCSEs in these subjects to the International GCSE (IGCSE).
"This shift in entries from GCSE to IGCSE may be as a result of changes made to GCSE English and English language from summer 2014," Ofqual said.
"The speaking and listening element is now reported separately and is not part of the final grade, and the written exams are now worth 60% of the qualification compared with 40% previously."
The regulator's report also shows:
:: There have been fewer entries from Year 11 students for each of the three sciences for this summer, with biology entries down 12%, chemistry down 11% and physics down 9%.
:: Entries for science and additional science GCSEs are up by 32% and 18% respectively.
:: Entries for Spanish GCSE in Year 11 have risen from 78,000 last summer to 83,000 this summer (up around 6.4%), while French GCSE entries are up by less than 1% and German is down by 2%.
Education minister Elizabeth Truss said: "Children should take their GCSE exams when they are ready.
"But a situation had developed where children were being speculatively entered for GCSEs in the hope they could 'bank' a C grade and then move on to other subjects.
"If they didn't get a C, they were entered again and again until they did. Often this was not being done in the child's best interests but to help schools' league table rankings. That cannot be right.
"It was essential we took action and today's figures vindicate our decision - many more pupils are now taking their GCSEs at the right time, having studied it fully, and when they are likely to achieve the best grade they can."