Some schools are more than 50 per cent full during lockdown despite being open to key workers' children only.
Key workers claim they are struggling to drop their children at school where non-key worker parents are needlessly dropping off their children.
Under Covid lockdown rules, primary and secondary schools must remain closed until at least mid February.
Children with at least one key worker parent are permitted to attend class while others should engage in remote learning at home.
But some teachers and parents say schools are dramatically busier than the first lockdown closure in 2020.
And some key workers say they their child has been denied a place as classrooms fill up during the UK's third lockdown.
Geoff Barton, general secretary of the Association of School & College Leaders, told the Daily Mail: “We are hearing reports that attendance in some primary schools is in excess of 50 per cent because of demand from critical workers and families with children classed as vulnerable under criteria which has been significantly widened."
The key worker category has been significantly widened since the first lockdown, and now even includes Brexit transition staff.
On Wednesday, Ministers confirmed children can also learn on-site if they lack the appropriate technology to learn remotely.
But parents have been accused of sending their children to school by falsely claiming key worker status.
It comes after the Department of Education also expanded the categories of vulnerable children who can still attend school.
Mr Barton added: "We are urgently seeking clarification about the maximum number who should be in school."
Health experts say that school closures are crucial in reducing the infection rate, despite the impact on children's education.
But reports suggest non-key worker parents continue to send their children to class, despite government rules.
Examples of busy schools include Willerby Carr Lane Primary School where half of pupils are attending lessons in East Riding, Yorkshire, according to Mr Barton.
Some 187 children arrived at the school on Wednesday and were placed in 'super-size bubbles' ranging from 25 to 32 pupils.
Meanwhile five times as many pupils are still attending class compared to the previous lockdown at Invicta Primary School in Deptford, south east London, he adds.
NHS worker Victoria Pass told of the difficulties she has faced in sending her children to school.
Speaking to Manchester Evening News, she said: "I’m a frontline NHS worker and been declined by my child’s school.
"Now I’m questioning whether to go to work or stay at home and not help save lives because my children need me."
Others are now choosing to keep their children at home because of the higher attendance.
Katie Cowley, a teacher from Rochdale, Greater Manchester, said: "I was amazed at the amount of children in when I dropped mine off.
"As a teacher myself I have no choice but to send them in. I am genuinely worried for their safety and may now decide not to send them in, I am so worried for them and myself."
The Department of Education updated its guidance last night amid growing fears of busy classrooms.
It states: "Children with at least one parent or carer who is a critical worker can go to school or college if required.
"Parents and carers should keep their children at home if they can."
Prime Minister Boris Johnson said he is full of "optimism and fundamental hope" that he can start to lift lockdown by spring.
But he refused to guarantee that all children will be back in school before the summer holidays.
While Cabinet Minister Michael Gove admitted school closures will harm the lives of children.
