Children aged 12 and over should wear masks just like adults, the World Health Organisation and Unicef have recommended.
This is particularly important when a one-metre distance can't be guaranteed and when there is a high transmission in the local area, the global bodies say.
But their latest advice appears to contradict Department for Education guidance for September school openings in England, which says that face coverings are not needed where pupils and staff are mixing in consistent groups.
Latest evidence: Teachers told they have no increased Covid death risk
A Q&A section on the WHO website on the topic of schoolchildren and masks says: "WHO and Unicef advise that children aged 12 and over should wear a mask under the same conditions as adults, in particular when they cannot guarantee at least a 1-metre distance from others and there is widespread transmission in the area."
Coronavirus: 'Let teachers wear face masks'
This weekend it emerged that a school in Scotland had issued guidance for students and staff to wear masks to stop the spreading of Covid-19.
A memo from James Gillespie’s High School, in Edinburgh, said that from Monday, teachers and students “must wear face coverings indoors whilst moving around between classes”.
The school said the decision was based on feedback from students, parents and staff.
The DfE's guidance has already been criticised by school staff unions, with Unison calling on the government to allow school staff to wear masks, should they wish to.
Today Unison's head of education, Jon Richards, said that the latest evidence outlined by government chief medical officers showed that it was vital that school staff should be allowed to wear masks.
“This acknowledgement of the transmission risk between staff in schools underlines why it’s vital they should be able to wear face coverings," he said.
“It's still unclear why government guidance won’t allow them, when they’re recommended for other workplaces.
“Regular deep cleaning is also key. Schools need more money to pay for specially trained cleaners, rather than relying on other staff who’re busy enough already.
"More thought must also be put into ensuring there’s proper social distancing in place for staff who may be working in cramped conditions, such as school meal workers in small kitchens.
"No one wants to see schools shut down again after a few days because we haven’t done enough to put necessary measures in place."
Recommendations from the Data Evaluation and Learning for Viral Epidemics (DELVE) group, which advises government scientists, also were at odds with government guidance on face coverings.
In a report released in July, the group said that schools should consider the use of mask or face shields for teachers, parents and older students during the Covid-19 outbreak.