Teachers risk damaging their pupils' education and future learning by entering them too early for GCSEs, the head of the biggest exam board has warned.
The Assessment and Qualifications Alliance (AQA) has already begun researching the effects of early entry.
Andrew Hall, the board's new chief executive, said he was first alerted to the downside of the growing trend to enter pupils for exams at younger ages through examiners' reports from the first wave of new-style modular GCSEs this summer.
"A number of those have had comments in about ... the relative immaturity of the answer and the lack of depth and understanding," he said. "There is concern (among examiners) that this is the youth of the student and the lack of preparedness. I am sure it is appropriate for some students to take these exams early but I am pretty sure it is not for everybody."
Mr Hall fears the phenomenon of schools trying to get pupils over the grade C threshold as early as possible could discourage and scare off those who fail, and limit the knowledge and skills of those who pass.
"Is this the beginning of people just not getting the depth of learning they need at GCSE, by doing it too early?" he said. "It is time for the educational establishment to have a look at this before we go too far."
This summer, AQA alone had more than 25,000 entries from pupils aged 14 or younger for modules in the new style GCSEs. And that figure does not include the high-entry English, maths, ICT and science subjects where the revised exams have yet to be sat.
Mr Hall believes the modularisation of most GCSEs from this year could be accelerating the trend, because it allows teachers to put pupils in early for a single module to see how they are progressing.
Asked about the extent of the problem, he said it was "pretty much across the piece" and not confined to any one group of subjects.
Early entry is already well established in English and maths GCSEs, as this summer's results showed. In English, 9.5 per cent of the results were from 15-year-olds and under - an increase of more than 50 per cent from 2009.
Maths was even higher at 10.9 per cent of entries, an increase of 37 per cent. The Advisory Committee on Mathematics Education warned ministers that schools were putting the demands of targets and league tables before the mathematical understanding of pupils.
The committee's concerns partly prompted AQA's research project, which has already started to examine whether there is a significant difference between the standard of early and other GCSE entries.
Brian Lightman, Association of School and College Leaders general secretary, said: "The league tables have had a damaging impact on this because they give schools a perverse incentive to enter pupils early when they could have done better."
See The TES next week for an interview with Andrew Hall.