By Hannah Richardson
BBC News education reporter
Passengers have been left stranded after the UK flight ban
Schools are struggling to open as teachers and pupils have been stranded overseas by flight cancellations due to volcanic ash.
With schools and colleges returning after Easter, a handful are closed with many more coping with a skeleton staff.
The Department for Children, Schools and Families said heads had contingency plans to deal with such issues.
But teacher supply agencies say they are under intense pressure to make more temporary staff available.
Teacher supply agency Protocol Education said it had had four-fold increase in the number of calls for teachers at schools in south London.
Capita Education Resourcing said it had seen the number of bookings and calls requesting staff double.
The worry is if this goes on. We have got the exams season looming
Head teacher Woodcote High School
Capita's managing director Simon Taylor said: "I sympathise with the effect this is having on schools; losing one or two members of staff on any given day is difficult but some London schools have reported having more than a quarter of their staff missing.
"Fortunately we have a large pool of well qualified and experienced staff ready to fill in until permanent staff members can return to work."
And Randstad Education, one of England's biggest supply teacher providers, said calls were higher.
Schools closed due to absences linked to travel chaos caused by the ash cloud include Burntwood School in Wandsworth. But the school is hoping to be partially open on Tuesday.
Walnut Tree Walk Primary in Lambeth has seven teachers stranded abroad and is closed.
But it is hoping to re-open on Tuesday, using a combination of supply teachers and senior staff.
Hanham Abbots Junior School, in South Gloucestershire, is also closed. It has said eight members of staff stranded abroad.
And a staff shortage forced Queen Elizabeth Grammar School in Derbyshire to close. Sixteen members of staff were unable to return to work because of travel problems.
Woodcote High School in Croydon has managed to open against the odds. Head teacher Mark Southworth said it had been planning to cope with staff absences since Thursday.
"We have 13 teachers struck abroad - we did have 14 but one intrepid member of staff made it in this morning, coming all the way back from Barcelona.
"He was the hero of the staff room and he got a big cheer when he came in."
He said the school had managed to open by calling in favours, asking staff and senior leadership teams to take extra classes.
"The worry is if this goes on. We have got the exams season looming and we have really got to protect those students doing their GCSEs and A-levels."
Some oral and practical exams are scheduled for this week.
The Joint Council for Qualifications, which represents the seven largest exam boards, said it was "monitoring the situation" but that some alternative arrangements were being made for individual centres where necessary.
Some schools are using the situation as a way of educating their pupils, basing assemblies and lessons on the impact of the volcanic cloud.
One teacher from a Bristol school, Parson Street Primary, who is stuck in Australia, used a webcam to lead a school assembly online.
Some 15 teachers and 40 pupils, from Orleans Park School in Twickenham, south-west London, who are stuck in Shanghai, China. But the schools has opened against the odds.
Head teacher Jo Longhurst said: "All the schools are in the same position and supply agencies are suffering from the same thing as well - they have staff stuck abroad too."
Meanwhile Gumley House School in nearby Isleworth is only partially open.
Some 200 15 and 16-year-olds students are also stranded in Hangzhou and Beijing.
Jerome Bertin's son is one of the teenagers stuck in Shanghai. He has been told he will not be able to fly back until 26 April
Mr Bertin said: "We have already looked to see if my son can get back overland and there is a train service from Beijing via Moscow, Copenhagen and Eurostar but it takes 10-12 days. He may have to do this sooner or later."
Teachers' union, NASUWT, has urged schools not to penalise teachers who fail to return to school because of flight suspensions.
General secretary Chris Keates said she had already seen one report of a council threatening to dock absent teachers' pay.
"This situation is affecting workers and workplaces nationally and internationally. To single out teachers is totally unreasonable and unnecessary," she said.
General secretary of the National Union of Teachers Christine Blower said there were schools where a "significant number of teachers" would be unavoidably absent.
"The result of this is that children may be sent home or, possibly, school closures.
"Schools should follow their cover policies which have been established to deal with this sort of issue, and try to employ supply teachers to cover classes for absent teachers."
The Department for Children, Schools and Families spokeswoman said it was monitoring the situation closely.
"Schools have contingency plans in place and head teachers are best placed to decide how to cover for absent staff effectively and support pupils forced to miss classes."
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