The usual caveats apply when it comes to these unnamed sources out of North Korea because it is so hard to independently verify what they are saying. This source could just be telling a journalist looking for a click bait headline what they want to hear:
An artificial earthquake caused by a North Korean nuclear test in September reportedly caused buildings to collapse and killed scores of people, including schoolchildren, South Korean media reported this week.
On Sept. 3, North Korea conducted its sixth nuclear test, successfully detonating a hydrogen bomb — one that could fit onto an intercontinental missile (ICBM).
The blast produced two shallow earthquakes in the Punggye-ri region, where North Korea’s nuclear test facility is located, U.S. and Chinese government seismologists reported at the time. Authorities in Japan, South Korea and numerous nongovernment experts in the United States confirmed that the earthquakes were likely the result of a nuclear test.
An unnamed source, who recently visited a village about 8 kilometers from Punggye-ri, described the damage to the South and North Development (SAND), a research institute that works with defectors from the North, according to the South Korean newspaper Chosun Ilbo.
The source said houses and a school collapsed in the village of Sindong-ri and that dozens of people were killed and injured, the paper reported.
“September 3 was a Sunday, but some 150 students were waiting in their classrooms to do some work,” the source said, according to Chosun Ilbo. “Casualties occurred when half of the school building crumbled.” [Voice of America]