Terrorist group Boko Haram has escalated its campaign of terror in northeast Nigeria, most recently with the abduction of more than 100 school girls.
Although all but eight of the 129 girls were eventually freed, the group remains a potent threat.
Much of group's violence has taken place in the Nigerian states of Adamawa, Borno and Yobe. In response, Borno's government closed its 85 secondary schools and sent nearly 120,000 students home.
Soldiers guarding the school targeted this week, in the town of Chibok, were attacked by the heavily-armed terrorist group as it abducted the girls.
"The targeting of schools and schoolchildren is a grave violation of international humanitarian law,” Ban said. “Schools are, and must remain, safe places where children can learn and grow in peace.”
Bloodshed between Muslims and Christians in Nigeria has killed more than 3,000 people — 400 in 2013 alone — since Boko Haram gained prominence in 2009.
Nigeria is listed as the 14th-worst country in the world for Christian persecution, according to the human rights organization Open Doors.
UPDATE: Officials from the school where the girls were abducted have cast doubt on the Nigerian military's report that most of the girls had been freed. Just 14 of the more than 100 girls abducted had returned -- four escaped from a truck and 10 more escaped after being "asked" to cook.
This story was published at 9:00 a.m. and was updated at 3:51 p.m.