England's exams watchdog, Ofqual, is to set out its initial findings on the row over English GCSEs.
Head teachers urged the exams regulator to investigate when it was revealed that grade boundaries for the exams were changed part way through the year.
Many pupils who were expecting a crucial C grade were given a D as a result.
This year GCSE results showed the first fall in the A* to C grades since they were introduced.
The National Association of Head Teachers wrote to Education Secretary Michael Gove and Ofqual chief executive to call for an inquiry.
'Quickly, but thoroughly'Ofqual's Glenys Stacey responded saying there were "questions" over how grade bands were set, adding that this would be done "quickly but thoroughly".
The Association of School and College Leaders (ASCL) also said it was "not afraid" to take legal action against exam boards over the grade boundary reform.
It came after exam boards acknowledged that boundaries had been moved by as much as 10% part way through the year.
Head teachers and a leading academy chain suggested certain groups of students could have been particularly affected by the changes.
Ms Stacey pledged to look at the detail of grade C boundary changes to ensure confidence was "maintained" in the examinations system.
A*-C grade importance
- This year schools in England have to ensure that 40% of their pupils reach the government benchmark of five A*-C GCSEs, including Maths and English
- This is tougher than last year's requirement, where schools were expected to ensure 35% of pupils made this grade - 107 schools failed
- Grades A*-C are crucial for pupils secure places in further education
- Also valued by employers and those offering apprenticeships
Figures supplied by ASCL suggested that a quarter of all secondaries in England and Wales had been affected, with complaints from elite independent schools starting to filter through.
