Matt Hood, principal of the online Oak National Academy, said early years teaching was "sophisticated" and it was "unreasonable" to expect parents to master it.
He said we may see the impact when pupils sit GCSEs 10 years from now.
The government said more than £1bn was being spent to help pupils catch up.
Mr Hood, from Penrith in Cumbria, said there had been a six-month disruption in schooling, adding: "The year groups I'm most worried about are reception and Year 1.
"I cannot stress enough how critical those early years are, focussed on learning sounds, how to read and count, those things are so fundamental.
"They are some of the most sophisticated things to teach, it's not something you can reasonably expect any parent at home to become an expert in teaching."
Mr Hood, an economics teacher who is also chair of governors at Bay Leadership Academy in Morecambe, said teachers were experts at identifying variations in an individual pupil's knowledge and abilities and plugging gaps.
He said: "What we are going to see is that variation being significantly more complicated because what the teachers do not know is the extent to which every pupil has completed all their work in the way they would have in the school building."
The priority when full schooling resumes would be to "identify gaps" with each child, Mr Hood said.
"The big risk is we create a never-ending cycle of pupils needing to catch up.
"One of the worst things you can do in most cases is take them out of their lessons to catch up. We want to them to spend their time with the most expert person they can, and that is their teacher."
Mr Hood has also worked with data companies to allow pupils free access to educational websites, as well as promoted drives to get internet-enabled devices to all children so "disadvantaged" pupils do not miss out further.
A Department for Education spokeswoman said: "The government will work with parents, teachers and schools to develop a long-term plan to make sure pupils have the chance to make up their lost education."
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