Jets will be grounded across South Korea and anxious parents will pray while their children take annual exams that could lead them to one of the country's top universities and eventually a good job for life.

As well as prayers at churches and temples in this country of 50 million people, the 690,000 students who sit the exams on Thursday have been boosting their chances by eating toffee, to help the right answers stick, and staying away from bananas and seaweed, that might make them slip in the tests.

"I have been so stressed just looking at other mothers send their children off to good colleges," said Kwon Jeong-hee, whose son is taking the so-called CSAT tests for the second time.

Kwon was praying at the Jogyesa Buddhist temple in downtown Seoul, which has held special prayer meetings for parents of CSAT exam takers. Many anxious parents have been praying for weeks, if not months.

"I haven't allowed guests into my home recently because of superstitions against strangers, and I don't let my son eat seaweed soup because it's unnerving," she said.

The exams are a major event here, and society scrambles to make things easier for stressed students.

During oral tests, aircraft will be banned from taking off and landing, and drivers are forbidden from sounding their horns. Police vehicles will even escort late-running students to the exam rooms.

Even the stock exchange will open an hour late to reduce the chance that students will be caught in traffic en route to the exams, an annual rite of passage that can literally make or break the lives of the 18-year olds sitting them.

"The mothers are more anxious than the children," said Yu Mi-ran, who has prayed daily at her church in central Seoul for over 20 days for her daughter.

Along with toffee, students are given presents of forks to help them "stab" the correct answers, while toilet paper is also good luck as in Korean it is called "pul-da," a homonym for "solve" or "unravel."

Porridge is also a banned food for test takers as "cooking porridge" in Korean is also slang for "messing up."