Millions of students headed back to class in France, Belgium and England on Tuesday as European schools cautiously reopened amid rising coronavirus cases in several countries, with face masks often mandatory.

Officials have drawn fire from parents and teachers worried that strict social distancing and other protective measures will not be enough to prevent a second wave of Covid-19.

But many governments insist that the greater risk is young people  losing out on crucial in-person lessons, and that keeping kids at home for distance learning puts too big a burden on working parents.

"I do not underestimate how challenging the last few months have been, but I do know how important it is for children to be back in school, not only for their education but for their development and well-being," Britain's Education Secretary Gavin Williamson said.

The UN's education agency UNESCO warned that just half the roughly 900 million primary and secondary students restarting school from August to October will actually be allowed back in classrooms.

"Several generations are facing the threat of school closures, which concern hundreds of millions of students and have lasted many months," the agency's director general Audrey Azoulay said in a statement late Monday.

In France, some 12.4 million students returned Tuesday, with masks required for all teachers as well as students over 11.

"It doesn't bother me to wear a mask, even if it does feel a little weird," said Marie, who was starting her first year of middle school in the southern French city of Marseille.

But many teachers were less enthusiastic. "How can we connect with children when half your face is hidden behind a mask?" said Julie Siata, who teaches English at another Marseille school.

'No zero risk' 

Pupils also returned Tuesday in Belgium, which has suffered one of the highest rates of coronavirus deaths in Europe.

Masks are required for those aged 12 and older, and must be kept in a protective case or pouch.

"You can't risk having the mask contaminated when taking it off to eat," said Martin, soon to be 13, as he headed to school in Brussels, adding that he was "stressed" about the new protective measures.

In England and Wales, where openings as well as start times are being staggered this week to avoid crowds on public transport and playgrounds, teachers are urging parents to avoid lingering with other parents after drop-offs.

The British government has faced a storm of criticism over reopening schools, after reversing course last week to announce that face masks would be recommended after all to stem a rise in new Covid-19 cases.

German schools reopened last month, as did many in Scotland which has control of its school system.

Masks will also be compulsory in Greece, where children are expected to return to school on Monday, while Spain will require all students 6 and older to wear masks when classes resume next week.

Children in Spain should also maintain a distance of 1.5 metres (5 feet) from each other, and are being urged to wash their hands at least five times a day.

"I believe fathers, mothers, and the education community can be sure that their sons and daughters, that school employees, will be much safer in schools than in other places," Spanish Prime Minister Pedro Sanchez told Cadena Ser radio.

"But there is no zero risk," he acknowledged, as Spain also reported an alarming surge in the number of coronavirus cases in recent weeks.

In Italy, where the virus first struck in Europe, concerns are growing that school reopenings set for September 14 could prove too risky despite masks and staggered opening and cafeteria times.

Three regions in southern Italy have already pushed back openings to the end of this month.