A sumo training hall of Nihon University's College of Engineering that was used by Nihon University Tohoku High School's sumo club is seen in Koriyama, Fukushima Prefecture, on Dec. 18, 2016. (Mainichi)

KORIYAMA, Fukushima -- The principal of a high school here where a teacher hit a member of a sumo club with a hammer said at a press conference on Dec. 18 that he had underestimated the problem, as the school announced the same day it views the situation seriously and will consider punishment for the teacher.

The school, Nihon University Tohoku High School, also said it would order the teacher, a man in his 20s, to stay at home from Dec. 19 to reflect on his actions. The school had allowed the teacher to keep his class and continue teaching for around five months even after confirming that a club member had been hurt in the violence from the teacher. The announcements mark a shift in the school's response following media reports on the incident.

At the emergency press conference on Dec. 18, principal Hiroyuki Matsui apologized for the delay in considering punishment for the teacher and in investigating the violence, saying he had "underestimated" the problem. "This was due to my lack of ability to give guidance (to students)," he said.

According to the school, the man began teaching at the sumo club in April 2015. During sumo practice, he repeatedly was violent toward a first-grade club member, including hitting the student on the head with a hard rubber hammer. In May this year the teacher injured the student with a deck brush while the student was naked during post-practice bathing. At the end of July the student transferred to another school. A man in his 50s who was coach for the club and who left the job at the end of September also threatened the student with a saw while the student was doing pushups during practice.

After being contacted by the student's parents, in July the school questioned the teacher. While the school judged the violent behavior as "excessive," it only went as far as handing down minor punishments like having the teacher submit a reflection note on what he had done wrong.

At the press conference, principal Matsui said the school had initially only handed down light punishment because he was "a teacher with potential." When asked about violence against other members of the sumo club, Matsui gave vague answers and said the school would check as it continued looking into the issue.

Hiroyuki Matsui, principal of Nihon University Tohoku High School, is seen at a press conference on Dec. 18, 2016. (Mainichi)

In a related development, it has been learned that the school on Dec. 19 indefinitely put the sumo club's activities on hold. According to the school, the principal gathered the three club members in his office in the morning and told them about the suspension of activities.

According to the Fukushima Prefectural Government, after a 2012 incident in which a boy on the basketball team at Sakuranomiya High School in Osaka took his own life because of physical punishment from an instructor for the team, the Ministry of Education, Culture, Sports, Science and Technology issued a notice to all prefectural governments in 2013 calling for the complete end to physical punishment. In August that year the prefectural government called on private elementary, junior high and high schools in the prefecture to report on physical punishment and how they did and will respond to it.

However, Nihon University Tohoku High School, a private school, did not report to the prefectural government about the violent behavior by the teacher during sumo practice despite learning of it in July.

Takuya Okazaki, head of the prefectural government's department on private schools and corporations, says he views the teacher's violence as physical punishment and it was "something that should have been reported." Principal Matsui, meanwhile, says he had "judged it wasn't necessary" to report the case.

Also on the morning of Dec. 19, the school held a school-wide gathering, where the principal apologized and said the school would take anti-violence measures. The school also plans to hold a meeting for parents.

One 18-year-old student on his way to school said of the incident, "I think it's really awful."