It is understandable that there is "anxiety" over exam grades, the prime minister has said, as pupils prepare to receive estimated results this week for tests cancelled during lockdown.
Visiting a school in London, Boris Johnson said he was also "very keen that exams should go ahead as normal".
A-level results in England, Wales and Northern Ireland are due on Thursday.
Scotland's First Minister Nicola Sturgeon has apologised for the handling of its exam results.
She acknowledged "we did not get it right" after results estimated by teachers for cancelled exams were downgraded.
The Scottish Qualifications Authority lowered grades using an algorithm - with pass rates for pupils in deprived areas downgraded further than those in more affluent parts.
Ms Sturgeon said her priority was to resolve the concerns about how some results had been downgraded, following protests by pupils.
With pupils in England, Wales and Northern Ireland awaiting A-level results this week, Mr Johnson said on a visit to a school in east London he wanted their hard work "properly reflected".
"Clearly, because of what has happened this year, there is some anxiety about what grades pupils are going to get, and everybody understands the system that the teachers are setting the grades, then there's a standardisation system," he said.
"We will do our best to ensure that the hard work of pupils is properly reflected."
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Meanwhile, Education Secretary Gavin Williamson said there was little evidence of coronavirus being transmitted in schools and the plan to fully reopen England's schools in September was guided by the best science.
Government advisers have warned the nation may have reached the limit of what can be reopened in society safely.
But asked whether parents should brace for local closures to combat flare-ups of the virus, Mr Johnson said education was a priority.
"The last thing we want to do is close schools. Education is a priority for the country - that is simple social justice," he said.
Guidance on reopening schools has been published for England. There are also separate plans for Wales, Northern Ireland and Scotland, where schools are scheduled to return from Tuesday.
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