Headteachers have warned they could still mount a legal challenge over GCSE English, as Ofqual said that the qualification will not be re-graded.
The regulator admitted grade boundaries were higher in June than they were in January, but insisted it would be "inappropriate" to re-consider either of them. Instead, students who gained their English GCSE this summer will be given an extra chance to re-sit exams or re-submit 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 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." Re-visiting the January boundaries could mean lowering the grades of other students' assessments, which would lead to further concerns of unfairness.
ASCL general secretary Brian Lightman said: "We are actively considering what legal challenge would be a way of dealing with this, although we would much prefer to solve this through other means. What we are seeking to do here is get justice for young people who have been badly affected by systemic failures that are not of their making."
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.
Ofqual chief regulator Glenys Stacey said the regulator looked carefully at how GCSE English qualifications had been awarded this year: "We have spoken to exam boards and they have been very responsive. Recognising the strength of feeling, they will be offering early resits for students who sat the June units."
The National Association of Head Teachers (NAHT) said it was "extremely disappointed" in Ofqual's findings. General secretary Russell Hobby said. "Our call for results to be re-graded remains. We will press for the report outcomes to be reconsidered and a re-grading to be carried out with the utmost urgency, to bring to an end the uncertainty over so many students' futures."
Shadow education secretary Stephen Twigg said that Ofqual's statement did not address the situation and called for the Education Secretary to make a statement to MPs: "Mr Gove must get a grip on this situation and come before Parliament when the House of Commons returns next week to explain to the public how it came about that pupils could receive such different treatment from exam boards and to set out his plans for clearing up this mess."