Only one in every 28 teachers think their low attaining pupils are engaging well with online learning while schools are closed, according to research published as Nicola Sturgeon insisted she was doing everything possible to help.
A survey of 704 teachers, conducted by the University of Glasgow, found only 25 thought children who were not academically inclined were coping well with home schooling.
In comparison, 498 teachers agreed their brightest pupils were doing well, almost 20 times as many. Four out of five teachers who responded worked in Scotland.
The researchers concluded: "We should not lose sight of the fact that this still leaves a significant tail of 206 teachers whose high attaining students are not engaging well, but the contrast with low attaining students is particularly stark."
Ms Sturgeon insisted her government would "leave no stone unturned" in trying to ensure that pupils' prospects are not permanently scarred by the pandemic.
But one of Scotland's most senior educationalists described as "incomprehensible" her government's decision to suspend the collection of literacy and numeracy data in schools.
Lindsay Paterson, professor of education policy at the University of Edinburgh, told the Telegraph that the move means "we will never know" the full impact of the shutdown on pupils.
The row broke out as the Scottish Government published new guidance for education and teachers ahead of pupils returning to schools part-time on August 11.
It said they should focus on literacy, numeracy and health and wellbeing initially but Scotland's largest teaching union warned they were "up against it" to stage exams in 2021 after this year's diet was cancelled.
Larry Flanagan, general secretary of the Educational Institute of Scotland, told Holyrood's education committee: "said: "If you've got a 120-hour course and you know that pupils aren't going to get that amount of teaching, is it fair to present them with an exam based upon 120 hours of learning?"
The warning came amid mounting concern that the closure of schools will further widen the huge attainment gap between pupils from wealthy and deprived areas.
Experts have warned of a postcode lottery in the help children and parents are receiving from schools during lockdown, with some councils refusing to consider online teaching over security issues.
There is simply no justification for cancelling the collection of these key figures. The SNP is more concerned about avoiding scrutiny than offering pupils a proper education.https://t.co/gYr3HM4aMt— Jamie Greene MSP (@jamiegreeneUK) June 4, 2020
The Glasgow University study found teachers were twice as likely to say that their high attaining pupils were "likely to seek out independent opportunities for learning" at home than their least academically inclined children.
The brightest pupils were also deemed to be far more likely to have the right home environment, computer equipment and parental support.
Speaking at her daily briefing, Ms Sturgeon said: "I say that to all young people, but I absolutely recognise the disproportionate impact on young people who are living in more difficult, vulnerable or deprived circumstances, and we will leave no stone unturned in trying to ensure any impact you suffer is not greater as a result.”
Directly addressing pupils, she added: “I don’t underestimate the challenges here but I absolutely will do everything in my power to make sure the young people of today don’t pay a long term price for what they’ve had to live through, through absolutely no fault of their own."
She defended her government's decision to suspend the collection of the Achievement of Curriculum for Excellence Levels (ACEL) figures, which are usually key in assessing pupils' performance and whether the Scottish Government is meeting their target of closing the attainment gap.
The statistics, based on teacher judgment, report on the percentage of school pupils in P1, P4, P7 and S3 who have achieved the expected CfE level for their stage. They cover reading, writing, listening and talking and numeracy.
Ms Sturgeon said this was an "inescapable decision" due to practical difficulties created by pupils being absent from school.
"There are still tens of thousands of pupils who don't have a table to work at," says Larry Flanagan, stressing that inequity of home learning is not just about computers. @SP_EduSkills— Henry Hepburn (@Henry_Hepburn) June 5, 2020
But Prof Paterson said: "The present circumstances make it all the more important to monitor individual progress, or lack of it, so as to be able to intervene for those children who have been badly affected by the shutdown. It would then be important for us all to know whether the shutdown has affected people badly, and, if so, who these are."
He added: "In the absence of this information, we will never know what the impact of the shutdown has been on children’s learning, and so we will not know what measures have to be put in place to compensate for losses."
Meanwhile, Scotland's fair access commissioner warned that the closure of schools risks damaging the chances of deprived youngsters winning a university place.
In his third annual report, Prof Peter Scott said "there is a risk of a loss of momentum without the discipline of regular school attendance" for children from socially disadvantaged backgrounds.
He said more deprived families may not have the home space to study, books, learning materials and IT. In addition, he said parents "may also lack the same level of formal educational attainment and necessary self-confidence to ‘substitute’ for teachers."
Prof Scott also warned that the cancellation of exams may "bake in" differences between schools. Teachers are to estimate pupils' grades but these will be moderated by the Scottish Qualifications Authority, which can lower them in line with a school's historic performance.