The row over this year's GCSE English grade boundaries shows no signs of abating after Ofqual said the qualification would not be regraded.
The regulator admitted grade boundaries were higher in June than they were in January, but insisted it would be "inappropriate" to reconsider either of them.
Headteachers were considering whether to mount a legal challenge over GCSE English, despite students getting an opportunity to resit exams or resubmit work in November.
The Association of School and College Leaders (ASCL) said it was not "acceptable or practicable" to make the students resit. It warned it could still begin a legal challenge against grade boundary changes on the grounds that it had disadvantaged certain groups of students.
Ofqual's report into the GCSE English crisis, published on Friday, found that January's GCSE English assessments were "graded generously" but the June boundaries were properly set and candidates' work properly graded.
But it said that it would not be "revisiting" the June grade boundaries because it would "contradict our responsibility to maintain standards over time and make sure results are comparable year-on-year".
Revisiting the January boundaries could mean lowering the grades of other students' assessments, which would lead to further concerns of unfairness.
Concerns mainly centre around pupils who were expected to get a C but instead got a D, and that this could affect their chances of getting into sixth-form college, or gaining an apprenticeship.
ASCL general secretary Brian Lightman said: "It is wholly unacceptable to leave the students and their teachers to pick up the pieces of a problem they did not cause. These changes implemented mid-year, without valid and reliable processes, must be reversed and arrangements put in place immediately to ensure that this does not happen again in future examination series."
Ofqual chief regulator Glenys Stacey said the regulator looked carefully at how GCSE English qualifications had been awarded this year. "People were particularly concerned about the June grade boundaries. We have found that examiners acted properly, and set the boundaries using their best professional judgment, taking into account all of the evidence available to them."
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