Seoul – An annual hush descended upon South Korea Thursday as hundreds of thousands of students sat the crucial national college entrance exam, delayed for a week by a rare earthquake.

Educational authorities had to change four exam sites at Pohang, affecting about 2,000 candidates, after a 5.4-magnitude quake shook the southern city a week ago.

More than 60 minor aftershocks have hit Pohang throughout the past week, forcing 1,500 residents to stay in shelters.

In South Korea’s ultra-competitive society, the college entrance test plays a large part in defining students’ adult lives, holding the key to top universities, an elevated social status, good jobs, and even marriage prospects.

If serious tremors are felt during the exam, students in Pohang have been told to take shelter under their desks, or head outside.

More than 200 buses were on standby Thursday to take students in Pohang to 12 alternative exam sites in case the city is jolted during the test.

“I am concerned about the quakes but I am summoning up the courage to focus on the exam,” student Kim Han-Beom in Pohang told the Yonhap news agency.

Extraordinary measures are taken in South Korea to ensure nothing disturbs students.

All takeoffs and landings at South Korean airports are suspended for 35 minutes to coincide with an English listening test, and all planes in the air must maintain an altitude higher than 3,000 metres (10,000 feet).

The Transport Ministry said 98 flights, including 36 international ones, had to be rescheduled because of the exam.

Public offices, major businesses and the stock market opened an hour later than usual on Thursday to help ease traffic and ensure students arrived on time for the exam, which began nationwide at 8:40 am (2340 GMT).

Any students stuck in traffic could get police cars and motorbikes to rush them to the exam centres.

Outside test centres in Seoul, junior students waved banners and chanted encouragement as candidates entered exam rooms.

“After the exam was put off for a week, senior students’ anxieties have increased. That’s why we came here early in the morning to cheer them up”, Kim Eun-Ji from the Dukseong women’s high school told AFP.

More than 593,000 students are sitting the exam this year, down 2.1 per cent from a year earlier.