A mass for the victims was held at the church in Saint-Féliu-d'Avall, south-west France, on Sunday morning with a commemorative rally near the local council's offices later in the day.
Prayers were to be held in the evening.
Flowers and notes of condolence were left at the council offices by fellow school students, local families and sympathisers from elsewhere in the region, which is on the border with Spain's Catalonia region.
Five school students were killed in the accident, the most serious of its kind in France since 1987.
The lives of six of the 18 people injured are still in danger.
Christmas festivities at the school and the rest of the village have been cancelled and shops have taken down their seasonal decorations.
Level crossing barriers
Meanwhile, investigators from the major southern city of Marseille are trying to establish whether the accident was the result of the level crossing barriers failing to work.
"There are witnesses who say the barrier was closed and others who say it was open. We have not finished the investigation," state prosecutor Xavier Tarabeux said.
The bus driver, a 48-year-old woman who was severely hurt, insists that the crossing barriers were up at the time of the collision, according to her employer.
"We saw each other last night in her hospital bed and she was perfectly lucid," said a bus company official. "She told us she crossed [the railway line] confidently and calmly, with the barriers open and crossing lights not flashing."
A school student travelling on a bus behind the one that was hit told France 3 television that "the barriers were not closed and there were no flashing lights."
The train driver, however, has told investigators the barriers were down.
National rail operator SNCF said it was "shocked by the serious accusations" made against it "without any evidence" in a statement.
The barriers were "functioning normally", it said.
Both the bus driver and the train driver have been given toxicology tests that came back clean.
Experts will examine the barriers' mechanical parts, which at first sight indicate that they were down, according to the prosecutor.