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.