Teachers in schools where colleagues are stranded abroad due to the volcanic ash cloud must accept that the situation is so "rare" that the new rules of "rarely cover" mean they should cover lessons, Schools Secretary Ed Balls has said.
Speaking to a TES Connect webchat - which took place while the flying ban was still in force - Mr Balls said the circumstances were suitably extraordinary.
"We have rarely cover arrangements in place and the current situation with teachers stranded abroad is one of those times when, rarely, teachers will need to play their part in covering for those colleagues."
But the NUT warned heads not to continue using their staff to cover lessons as the week went on.
The union's general secretary Christine Blower said: "There are schools where a significant number of teachers will 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."
Its advice to local officers is that supply teachers or cover supervisors should be used.
Official guidance said cover supervisors should be used only to provide cover for regular teachers for restricted periods of time and that they should only oversee the delivery of that teacher's work.
The union said that if suitable cover cannot be found, then pupils should be sent home.
Jo Longhurst, head of Orleans Park Secondary in Twickenham, west London, began the week with 15 of her 68 teachers stranded abroad, including four senior staff on a school trip with 40 Year 10 and 11 pupils in China.
She was able to employ six supply teachers to help her two existing cover supervisors, but on Tuesday began a rolling programme of sending a year group home at a time.
She said: "On Monday we were a bit creative. Some of the children were missing, too, so we could double up classes. The staff were fantastic. The PE department was great and took on additional numbers of children. A lot of staff gave up non-contact time. With rarely cover, it is something they do not mind doing as a one-off."
With one year group gone, there are eight teachers free each day to take lessons.
"Rarely cover means we are allowed to use teachers to cover in an emergency situation. In the end, you have to have teachers and do the best you can. It has been quite tricky, but I have a really supportive staff. They have been brilliant."
Meanwhile in China, lessons began again for the students on the trip. Ms Longhurst said: "The students were getting a bit stir crazy. We have sent them some money to do some day trips and they have started revision lessons. Our Year 11s are getting quite anxious about the exams coming up."