Officials at the Daejeon Metropolitan Police Agency did not immediately release the personal details of the suspect, describing him only as a man in his late 20s.
According to police, the suspect waited for the teacher to step out of a classroom at Songchon High School before stabbing him and fleeing the scene, which according to officials suggests they were acquaintances.
Police and fire department authorities did not specify the teacher’s health condition.
The incident follows a separate attack on Thursday in which a man rammed a car onto a sidewalk and stabbed multiple pedestrians, injuring 14 people near a busy subway station in Seongnam, at the heart of a bustling leisure and business district.
Among the five people who were hurt by the car, at least two were hospitalised in critical condition. Among the nine who were stabbed, eight were being treated for serious injuries, according to Gyeonggi province fire department officials.
Police apprehended a 22-year-old suspect whose identity has not been made public. They did not offer any immediate information about a potential motive.
The attack in Daejeon, about 120 km (75 miles) south of Seongnam, came hours after President Yoon Suk-yeol called for “ultra-strong” law enforcement measures to restore faith in public safety after Thursday’s violence, which he described as a “terrorist attack on innocent citizens”.
Yoon called for closer monitoring of social media to detect threats, deploying more law enforcement officers for prevention and equipping them with better suppression gear, according to his office.
Thursday’s attack was the country’s second mass-stabbing case involving random targets in a month. The attacks have sparked fear in a country that has long been considered safe with a low murder rate and strict firearm curbs.
In July, a knife-wielding man stabbed at least four pedestrians on a street in the capital, Seoul, killing one person. Attacks by firearms are rare in South Korea, which tightly controls gun possession, but there are no meaningful restrictions applying to knives, including kitchen tools that are often used for attacks.