A top cram school in education-obsessed South Korea used tiny audio recorders and cameras hidden in pens to record exam questions and then passed them on to students, prosecutors said Tuesday.

They said the five bosses of Hackers Education Group, a test preparation school in Seoul, have been charged with breach of copyright by passing on the questions asked in standardised English proficiency tests.

From 2007 to 2011 Hackers sent its employees to take the English tests, including the TOEIC (Test of English for International Communication), and to smuggle out the questions, prosecutors said in a statement.

They said some employees memorised questions while others copied them by hand or used the hi-tech gear.

The school then allegedly violated copyright by posting the questions and answers on its website and distributing them to students at its classes.

The students would not necessarily face the same questions when it was their turn to take the tests, but would likely face similar ones. Media reports said Hackers had earned a reputation for accurately predicting test questions.

The tests were developed by the US Educational Testing Service (ETS).

"Such a practice has prompted the ETS to raise questions over South Korean students' genuine English-speaking ability... and sparked a negative international image of South Korea," the Seoul Central District Prosecutors' Office said in the statement.

Hackers admitted it used special equipment to record test questions and vowed not to do it again. But it said it had not violated copyright.

In a statement it said documenting the test questions was only part of legitimate research.