Ministers are being warned that schools will become a "cauldron of Covid" unless heads get more support to avoid further disruption this autumn.
Labour wants schools to have more flexibility over whether children wear masks in class or 'bubbles' are asked to self-isolate at home in the event of an outbreak.
Vaccines chiefs also face pressure to speed up their decision to approve jabs for 12 to 15-year-olds to prevent more turmoil.
Shadow schools minister Peter Kyle told the Mirror that ministers should "reopen" discussions with the jabs experts to explain the delay.
Millions of younger children in the United States and across Europe are already getting jabbed.
Get a daily morning politics briefing straight to your inbox. Sign up for the free Mirror Politics newsletter
School leaders and teaching unions have warned that heads have been left without sufficient tools to manage any Covid outbreak in their schools.
The Government scrapped the measures at the end of last term after hundreds of thousands of children were forced to miss class.
They handed over responsibility for testing and tracing to the failed Government scheme.
Mr Kyle said: "Next week, because of Government scrapping mitigation measures and the failure to vaccinate they have created a situation where schools will be a cauldron of Covid.
"This is so serious that if Covid does not tear through our school system in the autumn term, then it's acting in a different way than it has been over the last 18 months."
The Labour MP questioned how medicines' regulator the MHRA could declare the Pfizer vaccine safe for young teenagers but the JCVI delay in recommending it.
Ministers have asked the NHS to start preparing for the rollout of Covid-19 vaccines to all children aged 12 to 15 from September, pending approval.
Education Secretary Gavin Williamson has confirmed that parental consent would be required if vaccines chiefs do green light the roll-out.
But Mr Kyle suggested the decision should have been made before the summer holidays to allow time for young people to get jabbed - and stressed the urgency of acting now.
"Ministers should be reopening conversations with the JCVI about offering the vaccine. They need to be quizzed about their rationale for delaying," he said.
"All along the JCVI has been at the forefront of the vaccine roll-out globally.
"But with this cohort it is lagging months behind the rest of the industrialised world and not even having a national conversation about it."
Some members of the committee are understood to remain opposed to extending jabs to all younger teenagers when the move would primarily benefit adults.
Under current guidelines, 16 and 17-year-olds across the UK are eligible for the coronavirus jab alongside those aged 12-15 who have suppressed immune systems, severe learning disabilities or live in a household with those at high risk.
A No 10 spokesman said the Government was waiting for the JCVI's advice. "They're still looking at the data and waiting for more data to come through," they added.
Paul Whiteman, general secretary of school leaders’ union NAHT, told the Mirror: “Whether to vaccinate younger children is a decision for medical professionals to take. Schools should not carry any responsibility for vaccination promotion, enforcement or policing.
“The number of cases of Covid in schools is predicted to remain high, so disruption and missed education looks set to continue unless more action is taken to prevent transmission in schools.
“As ever, the Government needs to be clear about the rationale and scientific evidence that underpins whatever decision they make, in order to reassure families that they are taking the right course.”
Mr Williamson has urged parents to stop their teenagers getting "carried away" as they return to school amid fears of a new Covid surge.
The Cabinet minister said preventing a fourth wave of the disease was "not just a matter for schools".
The Government has promised that children returning to school in England will face far less disruption after two years of major upheaval to their education.
More than 7.3 million Covid tests are to be carried out on children in schools in the coming days - but after that they will be asked to self-test at home twice a week.
Mr Williamson argued that families have the biggest influence over children and urged parents to ensure youngsters were getting tested.
He warned that young people "throwing caution to the wind" this autumn as they resume normal life was a major threat to the recovery from the pandemic.
Scotland, where pupils have been back in classrooms for two weeks, is recording record numbers of coronavirus infections.
