Coronavirus year group "bubbles" proposed by the Government are "unworkable" as more than one million pupils use public transport to get to school, a think tank has said.
The Government set out its plans to keep children in class or year-sized "bubbles" when all pupils return to school full-time in an effort to limit the spread of coronavirus.
But an analysis by the Education Policy Institute (EPI) has said the plan is likely to prove "unworkable" as a "significant number of pupils" will mix with students from other year groups - and even other schools - on their journeys to and from school from September.
Researchers found that as many as one in three of all pupils at secondary school in England - approximately 1.1 million - use either a school bus or public transport to get to school.
The vast majority of pupils travelling on school buses or using public transport have no alternative, unless they have access to a private car.
The EPI said secondary schools typically attract pupils from up to three miles away - and in rural areas it can be significantly further.
The analysis concludes: "There is currently no credible solution to the problem of them then mixing with other year groups, and other schools, on the bus home again.”
Jon Andrews, author and deputy head of research at EPI, said: “The Government needs to be open about any assessment they have made of the risk that travelling to and from school poses."
The Government's safety advice for schools in England says students should keep their use of public transport to a minimum in the autumn.
Parents, staff and pupils will be encouraged to walk or cycle to school if at all possible - and schools have been told to consider using "walking buses".
On dedicated school buses, the "one-metre plus" social distancing rule will not be required - and schools have been told to consider whether pupils can be grouped in their "bubbles" on these designated buses.
Education Secretary Gavin Williamson told the Downing Street briefing on Thursday that the Government would be working with local authorities to ensure the necessary controls were in place on school transport.