At least six people were abducted by the rebels, who fled across the porous border into Congo after the raid on Friday night, according to the Ugandan military.
The school, co-ed and privately owned, is located in the Ugandan district of Kasese, about 2 kilometers (1.2 miles) from the Congo border.
The ADF has been accused of launching many attacks in recent years targeting civilians in remote parts of eastern Congo.
In March , at least 19 people were killed in Congo by suspected ADF extremists.
Ugandan authorities for years have vowed to track down ADF militants even outside Ugandan territory.
Persons:
Selevest Mapoze, Mapoze, Felix Kulayigye, Joe Walusimbi, Winnie Kiiza, Yoweri Museveni
Organizations:
Uganda People's Defence Force, Secondary School, Islamic, AFP, Getty Images, Authorities, Lhubiriha Secondary School, Allied Democratic Forces, Associated Press, AP, Twitter, ADF
Locations:
Uganda, Mpondwe, Congo, Islamic State, Ugandan, Kasese, U.S