In addition, 40 others were injured, two seriously.
Firefighters carry a survivor they rescued from the site of an avalanche in Nasu town, Tochigi prefecture on March 27, 2017, while Self Defense Force personnel look on.
The group had been taking part in a three-day training scheme for members of school mountaineering clubs.
But some of its facilities were made available for the high school mountaineering trip organized by local physical education authorities.
Meanwhile, a separate local government official said investigation would ensue and find out why the group was climbing during an avalanche alert. They were sent to a hospital and were confirmed dead Monday evening.
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NHK World meteorologist Robert Speta said, according to The Guardian, that Tochigi prefecture had taken on an increase in unusual snow in recent days, increasing the avalanche risk with higher temperatures.
So instead of climbing the mountain as planned, the board of education said, teachers chose to have the students practise trekking on the much lower - and presumably safer - slopes around the ski resort.
At least one teacher is missing.
"As soon as I felt it, I also saw something white roaring toward us", a male student who was caught in the slide told NHK in a phone interview.
The disaster agency said many others were injured in the avalanche.
The resort ended its season of skiing activities last week, and the weekend climbing event was scheduled to end several hours after the avalanche struck.
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