Free travel on London's transport for under-18s will be suspended after the October half-term, according to government plans seen by the BBC.
The plan to temporarily end free travel for 11 to 17-year-olds was a condition Transport for London's (TfL) £1.6bn lockdown bailout.
About 30% of children who currently travel by bus to school would still be eligible for free travel, TfL said.
The Mayor of London called on the government drop the plans.
It is hoped the move will aid social distancing during the COVID-19 recovery, and free travel for under-18s is expected to be re-introduced, though no date has been set.
A spokesman for Sadiq Khan said the policy "will hit the poorest Londoners hardest and place significant administrative and financial burdens on already stretched schools and local authorities".
"Rather than forcing this measure through, the Government should work with TfL to find a better solution that doesn't risk hitting the poorest Londoners hardest".
In a letter to TfL, Transport Minister Baroness Vere of Norbiton said the government plans to start the temporary suspension of free travel for 11 to 17-year-olds "immediately after October Half Term".
Half-term runs from Monday 24 October to Friday 28 October this year.
In order to reach this date, Baroness Vere said it is "vital" TfL continue to work "closely and collaboratively" with the Department for Transport.
Under the plans children aged 11-17 who live more than two miles from their school will still be entitled to Oyster cards allowing free travel.
Children with special medical or educational needs will also continue to travel for free.
Deputy Chair of London's Transport Committee Caroline Pidgeon called the proposal "misguided, complex and simply wrong".
Ms Pidgeon said: "The policy of free travel for young people that has existed since 2005 should not be altered.
"The last thing young people and low income households need is to face a further challenge this autumn."