German parents are turning to private tuition in unprecedented numbers to make up for teaching time lost through staff absences.
According to the Dortmund-based education lobby group LBE State, parents are investing more than 4.5 billion marks (pound;1.6 billion) a year in private lessons. In North Rhine Westfalia, one in five pupils now has outside tuition.
"It amounts to an increasing privatisation of education," said LBE State chairman Udo Beckmann.
The cause is frequent illness among the ageing teaching profession - the oldest within the developed nations, with an average age of 49 - and the fact that Germany does not have a proper system of teacher supply.
Schools apply to the authority for cover and rarely know if their request will be granted.
Other staff cover for absent teachers, and in primary schools children can be sent home without advance notice to parents.
"Neighbouring schools help us when their teachers are free," said Barbara Malichewski, headteacher of Hermann Loens primary school in Berlin. She must rework the entire school timetable almost every week. "Now the children are grateful when a teacher comes more than once."
In history, geography and art, there has been no teaching all term.