An Ethiopian official said "bandits" who crossed over the border from South Sudan were responsible, and said Ethiopian troops are pursuing the attackers.
Nearly 2 million school-age children live in South Sudan, but more than half don't attend school. Violence, displacement, destroyed schools and a shortage of teachers are all part of the problem.
The massacre reportedly occurred at a church compound, where the victims were forced into a shipping container. Amnesty International is calling it a "war crime."
A U.N. camp for displaced persons tells the story of South Sudan's woes. Its 120,000 residents, mostly kids, came to escape civil war violence and a growing food crisis.
U.N. chief Ban Ki-moon is urging both sides in the South Sudanese civil war to resolve their differences. In the meantime, some 2 million people are living in limbo in the brutalized nation.
"We are really sorry and sorry is an understatement," a U.N. spokesman told a South Sudanese radio station. The violence at the camp in Malakal, which is managed by the U.N., erupted on Feb. 17.
Two years ago, the United Nations made a spur-of-the-moment decision to open its bases to hundreds of thousands who sought refuge from war. Today, those bases resemble permanent communities.
Millions of South Sudanese have been displaced by two years of civil conflict. They've lost touch with family. Now they have a rare chance to call home.