Two drivers and four teachers died while 24 other students, aged between 10 and 12, were injured in the collision which occurred as the bus, carrying 52 passengers, returned from a ski trip in the Val d’Anniviers mountain region.
The exact cause of the horrific crash remains a mystery despite extensive investigation.
Top politicians from the city of Sierre joined the president of Valais’s cantonal government Esther Waeber-Kalbermatten and diplomatic representatives from Belgium and the Netherlands for the ceremony.
The ecumenical service took place at the Sainte Croix church.
In Belgium, the tragedy will be remembered at a ceremony on Thursday in Lommel, home to one of the schools involved, where a work of art memorializing the event will be unveiled.
At the St. Luc ski resort, where the students spent their holiday, a plaque with the first names of the child victims with an inscription in French and Flemish has been installed.
At the request of certain families, the names of the bus drivers were removed from the plaque, Le Temps reported online.
At 9.10pm on March 13th 2012, the bus hit a curb and collided with a concrete wall at the end of an emergency turnout area in the Sierre tunnel.
Rescuers recounted a "vision of horror", with the screams of children unable to exit the bus because of the extensive damage to the front of the vehicle.
Investigators believe the coach was not travelling at a speed in excess of the 100 kilometre an hour limit.
But it is still not clear why the bus veered out of the travel lane, with tests revealing that the driver behind the wheel was not drunk and did not suffer any sudden illness.
At the time, Belgian Transport Minister Melchior Wathelet said Toptours, the company that ran the coach, had an “excellent reputation.”
While most of the passengers aboard were Belgian citizens, six Dutch passengers were killed as well as a schoolboy with a dual Belgian-British nationality.