A further 24 children were injured in the horrific head-on collision in the Valais canton near the border with Italy.
The coach was carrying school children from Belgium who were returning home after a skiing holiday in Val d’Anniviers in the Swiss Alps.
Police said the children were mostly aged 12 and were from two schools.
There were 52 people on the coach. Both drivers of the coach were among the dead.
The victims were from Lommel and Heverlee in Flanders.
The wreckage of the bus after it crashed inside a motorway tunnel in Sierre, in the Swiss canton of Valais (AFP/Getty Images)
The coach, with a cream and brown striped logo, was travelling on the A9 motorway towards Sion and crashed into the concrete wall of a tunnel near the town of Sierre on Tuesday night at 9.15pm.
The head of the Cantonal police Christian Varone described the crash as “unprecedented”.
An investigation to determine the cause of the crash is underway.
Survivors were taken by helicopter or ambulance to hospitals in the area and to Lausanne and Berne.