A mix-up in the delivery of national tests has led to the postponement of science exams for some teenagers.
Children were sitting at exam desks when the problem came to light
Some schools in England did not receive enough test papers to stage the Key Stage 3 national science tests for 14 year olds on Tuesday.
Some delayed the start of the tests while they photo-copied the papers but others had to postpone the exam.
The exams authority, the QCA, has not said how many schools are affected but described it as a "small number".
One school which was affected was Virgo Fidelis Convent Senior School, in Norwood, South London.
A teacher there described what happened: "On the day of the exams, the scripts were opened in the presence of the pupils as stipulated by the board.
"The first paper was fine. The second paper was a disaster."
The teacher said when the bags with the second exam papers were opened in front of the children in the afternoon, some were found to have the papers for the exam which had been taken in the morning.
Others had the correct paper, but not enough copies. The school photo-copied the correct paper and the exam was taken an hour later.
The school telephoned 90 parents to tell them their children would be home an hour later than usual.
The test papers concerned were sent out by the National Assessment Agency, a subsidiary unit of the exams authority, the QCA.
A spokesperson for the QCA said: "The vast majority of schools received the tests that they ordered, but a small number did not receive all of the Key Stage 3 science papers they needed for Tuesday morning.
"All pupils in the affected schools will be able to sit the tests.
"Schools were given permission to make photocopies. If this was not practicable, they will be allowed to sit the tests later."
Schools are being encouraged to re-schedule the tests for as soon as possible and by Monday at the latest.