Two of the first schools in Germany to reopen following the summer holidays were forced to close on Friday after a teacher and a pupil tested positive for the coronavirus.
Authorities said there was no indication the infection had spread at either school but both were closed as a precaution.
Pupils and staff at a primary school were ordered to self-isolate for two weeks after a pupil tested positive.
Separately, a secondary school was closed for four days so staff could be tested after a teacher was found to be infected.
“We said from the start that there would be suspected cases in schools,” Bettina Martin, the regional education minister for Mecklenburg-West Pomerania said.
“As long as the coronavirus has not been eliminated and there is no vaccine, we have to deal with it. The protection of pupils and staff comes first.”
Schools have been open across Germany since May, after the coronavirus lockdown was lifted. Although several have closed for short periods after individual infections were detected, so far there has been no serious outbreak.
But many schools are returning to full class sizes for the first time following the summer holidays, and parents’ and teachers’ groups have expressed concern over the risk of transmission.
Before the summer break, schools in most regions divided classes into smaller groups so social distancing could be observed, but parents complained after that left many children only able to attend classes for a few days a week, and the German authorities have now ordered full classes to resume.
Summer holidays are staggered by region in Germany and schools in the north-eastern region of Mecklenburg-West Pomerania are the first to return this year.
The Ostsee primary school in Rostock is to close for two weeks from Monday after a pupil tested positive for the virus.
Classes were moved outdoors as a precaution on Friday until parents could pick their children up. Pupils and staff have been told to self-isolate at home for two weeks.
The Goethe-Gymnasium secondary school in Ludwigslust-Parchim, more than 60 miles away, was also forced to close after a teacher tested positive.
The teacher had not taught any classes since the summer holidays, and the school’s 800 pupils are thought to be unaffected.
But she had held meetings with fellow teachers and the school will remain closed until Wednesday next week so all staff can be tested for the virus.
Senior German virologists on Friday became the latest to join calls for facemasks to be worn at all times in schools.
“A lack of prevention and control measures could quickly lead to outbreaks, which could force schools to close again,” warned the German Society for Virology, whose leading members include Prof Christian Drosten, the chief advisor to Angela Merkel’s government on the crisis.
“We urge caution against the idea that children do not play a role in the pandemic and in the transmission.”
Anja Karliczek, the education minister, has previously called for facemasks to be worn in classes.
A study in Saxony, the only German region to resume full class sizes before the summer holidays, found no evidence schools play a major role in transmission.
Scientists from Dresden University tested 1,500 children and 500 teachers for the virus and found antibodies to the virus in only 12.
“Children may even act as a brake on infection,” Prof Reinhard Berner, the leader of the study said.