Heads of schools in the UAE said months of distance learning meant pupils needed to catch up fast.
They warned a gap in learning had developed that must be closed with pupils preparing for major examinations.
University offers will depend on the final marks pupils score and it is critical they cover the entire curriculum in the months ahead.
"Pupils regressed, especially those who have a slower pace of learning," said Muneer Chalil, principal at Darul Huda Islamic School, an Indian curriculum school in Al Ain. "Pupils in the primary section need one-to-one attention and were affected."
The school has more than 1,700 pupils and fees range from Dh5,000 in kindergarten to Dh10,300 in grade 12. Of this, around 200 pupils are at the school for in-person lessons daily while others join online. The whole learning process had "undergone a sea change in the past months".
The school follows the Indian Central Board of Secondary Education examinations, which reduced coursework by 30 per cent in higher grades to help pupils finish.
Children were away from face-to-face classes between March and July in the UAE. Many schools are yet to return and some had to close once again because of isolated Covid-19 cases. Pupils across the globe lost hundreds of hours of in-person classes as Covid-19 wreaked havoc on the world's educational systems. By May 2020, children in the United Kingdom had already lost around 20 days of schooling, while pupils in Estonia had lost close to 60 days of in-school classes.
Many working parents had to juggle helping to teach children, while also working from home.
"There was a pressure on parents ... especially with the younger children. It’s not like children were neglected but it hampered the remote leaning process," said Mr Chalil.
The social and emotional development of pupils in younger years has also been stunted after studying remotely for months.
"It has affected their personalities and their creativity. Six months for a kindergartener is a huge period," said Muneer Ansari, director at the International Indian School in Abu Dhabi. Mr Ansari said apart from coursework, pupils pick up essential communication skills at school.
"From a psychological point of view it has hit the pupils," he said.
Natasha Ridge, head of research at the Sheikh Saud bin Saqr Al Qasimi Foundation for Policy Research, said content was less critical in early years but social and emotional learning at school was more important.
She said pupils in exam years would face real challenges, especially those sitting for external examinations and boys particularly were being left behind in online education.
"Boys do much better with in-person instruction, especially the at-risk ones," said Ms Ridge. "They are really struggling to apply themselves while watching a screen all day as it is less interactive."
"Boys are at risk of disengagement; their cameras and mics are off. They look like they are present while they are on their phones and the teacher does not know if they are really focusing or not."
Jodh Dhesi, deputy chief education officer at Gems Education, the UAE's largest private school operator, said its headteachers had not seen children return after the summer with a knowledge gap, and that overall pupils had done themselves proud in exams and coursework.
"The best place for children to be at is school for social interaction," said Mr Dhesi.
"As we get pupils back, we hope more will return to in-person classes. We would love to have as many pupils in school as possible."
