Mount Sinabung has erupted repeatedly in recent days, pumping hot gas clouds up to three kilometres (1.8 miles) into the sky, the local disaster agency said.
The clouds of smoke shrouded the sky above a school in North Sumatra province but children continued to play outside in the yard in the shadow of the volcano.
Local authorities insist the school is safe as it is outside a seven-kilometre (four-mile) restricted area around the volcano, which has been erupting almost continuously since 2013.
Villagers also continue their precarious existences despite the eruptions, putting on masks to go shopping as ash falls from the sky, and tending to their crops.
Over 2,000 families have been evacuated from around the rumbling volcano in recent years, and many will soon be relocated to new villages after lengthy stays in temporary shelters.
Sinabung roared back to life in 2010 for the first time in 400 years. After another period of inactivity it began erupting again three years later.
In May last year seven people were killed in one of Sinabung's eruptions, while in February 2014 an eruption left 16 people dead.
Indonesia is home to around 130 volcanoes due to its position on the "Ring of Fire", a belt of tectonic plate boundaries circling the Pacific Ocean where frequent seismic activity occurs.
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