More than half a million South Koreans sat for the annual national college entrance exams on Thursday, pandemic rules adding stress to the eight-hour event seen as life-defining in Asia's fourth-largest economy.

This year's test-takers didn't face the delays and uncertainties of the first pandemic-era exams last year, but COVID-19 measures have left their mark on the College Scholastic Ability Test (CSAT) widely considered as indispensable for landing one of a limited number of jobs in a competitive society.

"I couldn't go to private institutes, study rooms, nor school properly due to coronavirus," said 17-year-old Ahn Jeong-min. "Still, I'm vaccinated, and everyone will wear face masks and use partitions during the exam, so I think I can take the exam well, feeling comfortable rather than much concern."

More than 509,000 high school seniors, graduates and others have signed up to take the single-day, five-session exam held at 1,251 test sites nationwide, according to the education ministry.

At least 173 people who tested positive for the coronavirus or otherwise required isolation will take the test at hospitals or separate exam centres, the ministry said.

Thursday morning saw traditional society-wide efforts to help the test-takers, with the country's financial markets opening an hour later than usual to ease traffic.

Commercial air traffic was scheduled to be suspended during a key period in the afternoon, warplanes from the South Korean and U.S. militaries will be grounded, and live-fire exercises shut down throughout the day, officials said.

"We're doing our part to keep distractions down so you can keep your scores up!" tweeted U.S. Forces-Korea (USFK), which includes about 28,500 American troops stationed in South Korea.

Pandemic measures meant other, louder traditions to wish the test-takers well were missing.

Outside schools in Seoul, there was none of the customary cheering by high school juniors, praying parents, or schoolmates who typically beat drums and hand out sweets to participants.

Lee Eu-gene, a mother who said she had an older child take the test last year, said her son sitting for the exam this year seemed to be better off because schools had more in-person learning.

"He studied in this situation, so it's in the mother's heart that I hope he will get good results and happily expand his future," she said.