Heavy downpours since Wednesday triggered flooding and landslides that hit several districts on Sumatra island.
At least 17 people were confirmed killed in the last three days in North Sumatra province, while five others were killed in West Sumatra, authorities said.
"Eleven students of an Islamic boarding school in Muara Saladi village, Mandailing Natal district, North Sumatra died Friday afternoon during class as they were struck by a building which was destroyed by the flash floods," spokesman for the national disaster agency Sutopo Purwo Nugroho said.
Dozens of houses have also been destroyed.
"The disaster and search and rescue agency have gone to the districts but have been hampered by the landslides in several areas," the head of North Sumatra disaster agency Riadil Lubis told AFP.
Landslides and flooding are common in Indonesia, a vast tropical archipelago prone to natural disasters and torrential downpours.
In February, 12 people died when an avalanche of mud and rock cascaded down a steep slope in central Java, Indonesia's main island.
In June 2016, nearly 50 people died when heavy downpours sent torrents of water, mud and rock into villages also in Central Java province.
Published : October 13, 2018
By : Agence France-Presse Mandailing Natal, Indonesia