There is not enough evidence to support removing face masks in classrooms after Easter, the leader of the UK’s largest teaching union has said.

Kevin Courtney, joint general secretary of the National Education Union (NEU), said face coverings should remain in secondary schools “until the science says that it’s safe to remove them”.

His comments came after millions of pupils returned to school in England.

Secondary school pupils are being advised to wear face coverings wherever social distancing cannot be maintained, including in class, but the Government has said the measure will be reviewed at Easter.

Mr Courtney said: “We’re worried that with only two weeks’ data, or two and a bit weeks’ data, it’s not safe to make that decision right now and therefore to say remove them after Easter.”

Earlier this month, MPs heard that school leaders had received “threatening letters” from parents who did not want their children to wear face coverings.

Speaking at a press briefing, Mr Courtney said: “It’s really important that we get every mitigation in place that we can to try and stop cases spreading in schools.

“Ventilation is going to get easier as the weather warms, but we really need to work on ventilation. We would be in favour of continuing mask wearing until it is clear that we don’t need mask wearing.”

But he added that they hoped masks will not have to stay until the end of the academic year.

A separate poll by Unison, of more than 7,000 school support staff, found that nearly two in three (63%) feel safer at work because of face coverings.

Jon Richards, head of education Unison, said: ​“Face coverings in schools have only been in place for a few weeks​. Lifting this measure before the impact on transmission in schools ​has had time to be assessed would be rash.”

He warned: “Removing the need for masks in schools could see a spike in cases. This is the last thing the country needs.”

Teachers in England will decide their pupils’ GCSE and A-level grades after this summer’s exams were cancelled.

Addressing the consequences of teacher assessment, Mr Courtney said: “Some secondary teachers are telling me that children are being more polite, more diligent and getting on with the work that they’re being set.

“You know there’s some backwash from the fact that there’s teacher assessment and that tells you something about an exam system as well.

“You shouldn’t be in a place where you can just do it all by cramming at the last minute. That’s not the way real life is. We need to have some places where children do some work during the course of the year and it matters.”

Delegates at the NEU’s virtual annual conference over the Easter break will debate a motion that calls for the abolition of GCSEs in the long term following growing calls to reform the national assessments.

Dr Mary Bousted, joint general secretary of the NEU, said: “We have to ask ourselves how long can we continue to have a system which promotes rote memorisation?”

During the briefing, Mr Courtney also called on the Government to advocate “more strongly” for families to take rapid coronavirus tests at home.

Secondary school pupils were asked to take three voluntary Covid-19 tests on site and one at home over the first fortnight. After this initial stage, they will be sent home-testing kits to use twice weekly.

Mr Courtney said: “As a parent, you find out that your asymptomatic child is positive. They were asymptomatic, they weren’t suffering, you find out that they’re positive, then you have to keep them home and you have to stay at home, so it’s not in your direct, immediate interest.

“But if everybody did that then we could control cases much better. We would like to see Government advocating for that use more strongly.”

Mr Courtney said the pandemic had revealed the “truly shocking” state of child poverty in England.

He said: “When our members have had to teach online, they’ve had the chance to look into children’s homes.

“And when you see that you can’t unsee it.”

“You see children who are in bedrooms that they are sharing with two or three other siblings, you see children with damp on the walls.

“You see children who are in rooms that aren’t bedrooms, they live in the living room,” he added.