There are growing concerns that the ongoing disruption caused by coronavirus this academic year could impact the 2021 cohort of students.
Unions have voiced their concerns to Education Secretary Gavin Williamson to demand changes to next year's school exams to ensure grades are "properly recognised and rewarded".
It comes in the wake of the government's 11th hour U-turn on GCSE and A-level grades after the controversial algorithm originally devised was roundly criticised.
Now the joint General Secretaries of the National Education Union (NEU) have said last week's chaos "must never happen again" and urged the minister to prepare for new spikes in Covid-19 that could lead to "further loss of schooling".
In the letter, the pair said: "It is clear to the National Education Union that government needs to make much bigger changes to next year’s exams in order to build confidence that the grades awarded, upon which young people’s life chances are determined, properly recognise and reward their achievements."
They stressed: "You should be working, now, to examine different possible scenarios and to develop contingency plans in case of further school and college closures."
Ministers in the Welsh government have already warned that next year's examinations might not mark a return to normal.
Health minister Vaughan Gething told a coronavirus press briefing that students may not be sitting A-levels and GCSEs next summer either, and there was "much to learn and much to do ahead of next year".
"We need to learn because next year we may not have a normal exam season either," Mr Gething said.
"I cannot look anyone in the eye and say that everything will be fine come next May and June".
Among the demands of the NEU leaders includes calling on the government to reduce the amount of content assessed in 2021’s GCSE and A-level exams.
The letter also stresses Number 10 should work with teachers to develop a "robust" system for moderated centre-assessed grades, and commission an independent review into the assessment methods for GCSEs and A-levels.
They added: "All options should be considered to ensure that young people are rewarded for their achievements, supported to fulfil their potential and not held back due to their background."
The Department for Education has been approached for comment.