Gunmen in Nigeria on Saturday released 27 teenage boys who were kidnapped from their school last week in the north-central state of Niger, while security forces continued to search for more than 300 schoolgirls abducted in a nearby state.

Schools have become targets for mass kidnappings for ransom in northern Nigeria by armed groups.

On Feb 17, 27 students, three staff and 12 members of their families were abducted by an armed gang that stormed the Government Science secondary school in the Kagara district of Niger state, overwhelming the school’s security...