Head teachers threaten legal action after the exams regulator refuses to regrade GCSE papers.
The threat comes after watchdog Ofqual acknowledged that grade boundaries had been changed part way through the year but insists papers will not be remarked.
Unions say thousands of pupils missed out on the results they should have got.
In a statement, Russell Hobby, head of the National Association of Head Teachers said: "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.
“There is still the potential for a legal challenge and we are working with other teaching unions to explore all options."
The Association of School and College Leaders (ASCL) said it was not "acceptable or practicable" to make the students resit exams.
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, released 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.
"Many will already have left their schools and decisions about apprenticeships and further courses of study are being made now as term starts and enrolment in most schools and colleges is already under way."
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."
The June boundaries have been properly set, and candidates' work properly graded, she added.
"The issue is not the June but the January boundaries. Again, examiners used their best judgment in setting these boundaries, but they had less data and information to work with."
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.
Overall, 719,000 submissions were made in the summer and 51,000 in January - about seven% of total entries.
:: The Department for Education said it would be discussing the situation with Ofqual in the coming days.