Heads' leaders have warned teachers against giving pupils the benefit of the doubt when assigning teacher-assessed GCSE grades this year.
But they say the use of teacher assessments because of the coronavirus outbreak is an opportunity to show that an "alternate universe is possible" for those opposed to the current GCSE system.
In guidance released today regarding the use of teacher-assessed grades for this summer's GCSEs, the Association of School and College Leaders (ASCL) advises schools not to give pupils the benefit of the doubt in awarding more grade 4s - considered a pass grade - than students would normally have achieved.
Coronavirus: GCSE and A-level grading plan 'by Easter'
Coronavirus: Teacher assessments will be used for exams
Coronavirus and schools: LIVE 30/3
"It is understandable that teachers might want to give more students the benefit of the doubt by, for example, awarding more grade 4s than they might otherwise have obtained," the statement from Duncan Baldwin ASCL's deputy policy director says.
It adds that "this would be wrong for several reasons", as Ofqual and exam boards would take steps to ensure grade distributions resembled previous years' results, and that "where centres produce grades which seem particularly generous, a moderation process will be applied".
"Assuming that things head back to normal for the 2021 cohort, an inflated picture in 2020 would disadvantage those students currently in Year 10 and Year 12," Mr Baldwin writes.
"They have enough to contend with having had their own studies disrupted."
"This is an opportunity for the profession as a whole to show that it can, and will, produce reasonable grades through the process of teacher assessment.
"For those who believe that the current system of examinations at 16 is inappropriate there is a chance here to show that an alternative universe is possible."
Mr Baldwin said there were "difficult ethical issues" associated with allocating grades, such as historic differences in the performance of girls and boys, and between disadvantaged pupils and their wealthier peers.
But he cautioned that "whilst we should as a profession be exercised about the inequalities which are present in the system, this is not the moment or the method to put it right".
Mr Baldwin also advises: "Teachers should ask themselves the question 'What grade would this particular student most plausibly have achieved if they were taking the exam?'.
"This is not to be confused with a target grade, which might have an inflationary effect if used instead."
ASCL said headteachers should use the Department for Education's data on national performance for their subject. Schools that tended to perform better than the average from year to year would be moderated accordingly.
The union also said teachers could use at www.ascl.smidreport.com to check how their school and subject performed against national outcomes.
The union said teachers should consider which grade would be most "plausible" if students had taken the exam, and that teachers should be considering whether the assigned grades look "fair".
"On a practical note, ASCL would advise teachers and leaders that they should not be seeking any further work from students at this point to support teacher-assessed grades," the statement said.
"Not all students will be able to respond; some will be ill whilst others will be living under more difficult circumstances at home.
"We believe the priorities for pupils in Year 11 and 13 are to ensure that they complete the curriculum in the subjects which they wish to pursue in the following year, not to add collateral to the assessment process."