* Parents fear the worst, prepare to fly to Switzerland
* Teacher, pupils among 28 killed in tunnel bus crash
* Prime minister mourns "sad day for Belgium" (Adds details in paragraphs 10-12)
By Philip Blenkinsop
HEVERLEE, Belgium, March 14 (Reuters) - At morning assembly on Wednesday, the children of St Lambertus primary school in the Belgian town of Heverlee learnt of the bus crash in the Swiss mountains that killed a teacher and several of their classmates.
The bus, carrying 52 people, hit the side of a tunnel on the way back from a school ski trip and slammed into a wall, crushing the front third of the vehicle.
Six of the 28 dead were adults and the rest children, most of them 11 or 12 years old.
Twenty-four pupils from the Catholic school in the Leuven suburb of Heverlee were on the ski trip, a popular annual event. A teacher and a trip organiser were killed in the crash along with eight local children.
"The eight sets of parents, they can only sit and wait, they just don't know. I'm in pain, I have tears inside," said Dirk De Gendt, a priest at St Lambertus Catholic church and a member of the school board.
"We don't have words, only deep grief. They were supposed to be back now."
The remainder of those killed and injured were from the Netherlands and from Lommel, a town which, like Leuven, is in Flanders, the northern, Dutch-speaking region of Belgium.
Parents began arriving at St Lambertus school in the late morning to prepare to fly to Switzerland on Belgian military aircraft. While some knew their children had survived with injuries, others had no confirmation and had to assume that theirs were among the eight children killed, police said.
Psychologists were travelling on the plane with them.
At the school gates, teachers posted dozens of pictures drawn by pupils during class on Wednesday honouring the dead.
Some depicted hearts, rainbows and tulips, others were drawings of their classmates and of the teacher, Mr Frank.
"We support you," read a note under one drawing. Another commemorated the children of class 6A and another said: "I hope we will wake up quickly and this nightmare will be over."
Two police officers stood discreetly by the gates of the school, receiving distraught parents, and wellwishers placed several bouquets of flowers at the entrance.
"There's no news, simply no news," said one red-eyed father as he arrived with an overnight bag to take the flight to Switzerland.
The head of Belgium's Catholic Church, Archbishop Andre-Mutien Leonard, spoke to some of the distraught parents as they arrived and expressed their quiet grief and desperation.
"Some of them don't know the fate of their children. There is terrible fear," he told reporters.
Heverlee is a small community that is home to several schools and campuses that form parts of the Catholic University of Leuven, one of Belgium's leading universities. The town is also the location of a Commonwealth cemetery from World War Two.
Many of Heverlee's residents work in nearby industries, including AB InBev, the world's largest brewing company, which is based in Leuven.
Most of the children killed were in the last year of primary school. For many, it will have been the first holiday abroad without their parents.
"It is a sad day for all of Belgium," said Prime Minister Elio Di Rupo, who was to travel to Switzerland later on Wednesday. (Reporting by Phil Blenkinsop and Robert-Jan Bartunek; Writing by Luke Baker; Editing by Robin Pomeroy)