SEOUL, South Korea (CNN) -- A South Korean man admitted to police that he killed a former school teacher who punished him for cheating on a test 21 years ago, authorities said Monday.
The man, who police identified only as Kim, told officers that he had repeatedly sought an apology from the teacher for the 1987 incident -- but was rebuffed.
At the time, Kim was a 10th grader, the investigating detective at Seoul's Eunpyeong police station said.
He told police he had finished a test about 20 minutes early and decided to take a nap. When he awoke, he looked around at his fellow test-takers.
The teacher, a 58-year-old man identified by police only as Song, accused him of cheating and gave him a severe beating.
Kim told police that the teacher beat him about 100 times.
Last January, an unemployed Kim began asking the teacher for an apology, calling him on the phone and showing up at his school, police said.
Angered at being ignored, Kim told police he confronted Song outside the his house in the Eunpyeong district Sunday night and killed him.
Police arrested Kim at a hospital where he had gone to seek treatment for hand wounds that he suffered during the attack, officials said.
Authorities will decide later Monday on whether to charge Kim in connection with the teacher's death.
CNN's Kathy Paik contributed to this report