The exams crisis sparked by the Covid-19 outbreak "could easily be repeated next year", teachers have warned.
It is important to devise a "fallback" plan for GCSEs and A levels in 2021, as "there has to be really high probabilities of lots of children not being able to do the tests", according to the NEU teaching union.
Speaking at a fringe event at the Lib Dems' annual conference at the weekend, Kevin Courtney, NEU joint-general secretary, said moderated teacher assessment "can work", if there is "some clarity over what evidence teachers would need to produce".
"The crisis this year could easily be repeated next year, and we have all sorts of problems," he said.
"Teachers now – GCSE, A-level teachers – do not know what subjects best to teach because they still don't know whether they are trying to teach the whole of the curriculum to make up for the gap – which would be crazy, if that's what happens, because it will be really discriminatory – or if they can focus in on some particular areas of the curriculum.
"We think it's vital that children are assessed on what they have been taught, not on what they haven't been taught."
He added: "We think there has to be really high probabilities of lots of children not being able to do the tests, or of missing teaching to such a great extent, that we need a fallback position to the exams. We certainly need a fallback.
"And the fallback this year of teacher assessment didn't work...because they were rationing via the algorithm – that really didn't work.
"But teacher assessment can work – moderated teacher assessment – if there is some clarity over what evidence teachers would need to produce. There is a route down that way, and we need to start talking about it because it's likely to happen for some children and we're just not being clear about it."
Schools minister Nick Gibb said at the beginning of this month that the government would make a decision "very soon" on whether or not exams would be delayed in 2021.
A Department for Education spokesperson said: "We expect exams to take place next year and continue to work with Ofqual and the exam boards on our approach, recognising that students will have experienced considerable disruption to their education in the last academic year.
"There are a range of measures proposed by Ofqual following a public consultation, including a possible short delay to the exam timetable and subject-specific changes to reduce pressure on teaching time.
"We will continue to work with school and college stakeholders, Ofqual and the exam boards, to ensure that exams in 2021 are fair."