Students who are not happy with the GCSE and A-level grades they are awarded this summer will be able to sit exams in all subjects in the autumn.
This summer's exams were cancelled due to the Covid-19 outbreak and school closures, and so students' results will be based on teacher assessment.
Now England's exams regulator, Ofqual, says a "full suite"of exams - not just core subjects - will be available.
But head teachers are concerned about the practicalities of the announcement.
They say accommodating a full set of autumn exams alongside getting all pupils back to school in September will be a "huge challenge".
In a statement issued on Tuesday, Ofqual said students would be awarded calculated grades this summer, "giving them the opportunity to move on to further study or employment, despite the cancellation of exams".
"Students will be able to appeal - through their school or college - if they believe the process this summer was not followed correctly in their case; and centres themselves can appeal if they believe something systemic has gone wrong in relation to their results.
"Any student unable to receive a calculated result, and others who would like to improve their grades, will have an opportunity to take the full suite of GCSE, AS and A-level exams in the autumn."
The watchdog says that following the autumn exam season, exam boards will have to issue replacement certificates for the summer results - "if students request this", meaning students can keep the summer grade if it is higher.
Boards will also be expected to have the usual arrangements for reviews of marking and appeals.
'Managing the risks'
But Geoff Barton, general secretary of the Association of School and College Leaders, said there were practical issues to address.
"We are concerned about how schools and colleges will be able to accommodate and manage a full suite of autumn exams alongside the huge challenge of bringing all their pupils back in September, identifying learning gaps, and putting catch-up support in place," said Mr Barton.
This would have to be done while managing the risks associated with coronavirus, he said.
"We had argued for the autumn series to be restricted to A-levels, and GCSE English and maths.
"However, we understand the pressure on the government and Ofqual to provide the option of a full suite of exams in the event that pupils and parents are unhappy with centre-assessed grades in August."
Mr Barton said families should be assured that the process by which schools assessed grades this year is "robust".
"These qualifications will be every bit as valid as in any other year and will allow young people to progress to the next stage of the lives without hindrance, and without the need to use the autumn series."
Paul Whiteman, general secretary of the NAHT school leaders' union, said while he supported offering exams for all of the GCSE, AS and A-level subjects this autumn, schools might need other "hub centres" to accommodate exams.
"An exam series which runs through October and November will be very difficult for schools and colleges to manage, in a term where they be focused on their current students and mitigating the impact of the coronavirus pandemic on their pastoral wellbeing and academic progress."
Mr Whiteman added: "The government and Ofqual must act to ensure that schools and colleges are given significant support - including the options of setting up local hub centres - to be able to meet the needs of students wanting to take these exams, as well as focus on what is needed for their current students."