Most of the children were 11 and 12 years old, said Joren Vandeweyer, a spokesman for the Belgian Ministry of Foreign Affairs. The bus was on its way back to Belgium from a ski trip. It slammed into a highway tunnel in Sierre in the Swiss canton of Valais.
The bus was carrying 52 people in total: two bus divers, four other adults and 46 children.
The cause of the crash is under investigation. Belgium made available two military planes to take parents to see their injured children and to bring them back, Vandeweyer said.
"This is a tragic day for Belgium," Belgium's Prime Minister Elio Di Rupo said, adding he will be traveling to Switzerland Wednesday.