The ISC physics practical had to be extended by half an hour on Thursday because of alleged “confusing instructions” about a compulsory question.

The question, carrying nine marks, asked the examinees to carry out an experiment in electricity using a certain voltage. The confusion stemmed from the fact, examinees said, that the experiment could not be performed with that voltage.

The three-hour paper started at 9am.

“In some schools, teachers asked the examinees to do the experiment at a suitable voltage. At others, the students were asked to take the readings till a lower resistance to match the voltage,” a school principal said.

Some school heads explained the difficulties faced by the students to the Council for Indian School Certificate Examinations, which conducts the ISC and ICSE exams.

Around 10.15am, the council sent a text message to all school heads telling them that the test duration could be extended by half an hour. A few minutes later, the council sent another message saying the examinees should be allowed to take the readings till a suitable resistance value.

“There was no mistake in the paper,” said Gerry Arathoon, the chief executive and secretary of the council.