* Twenty-two children, six other people aboard killed
* Most of dead were Belgians, the rest Dutch
* Most children aged about 12, on way home from ski trip
* Cause of crash unknown, 3 theories probed
* Distraught parents flown on military plane to Switzerland
* Obama sends condolences; mass planned in Swiss city of Sierre on Thursday (Adds Obama statement, new quote from rescue official, other details)
By Robin Bleeker and Philip Blenkinsop
SION, Switzerland/HEVERLEE, Belgium, March 14 (Reuters) - Distraught parents flew to Switzerland on Wednesday after a bus carrying a Belgian school group home from a ski trip crashed into the wall of a Swiss tunnel, killing 22 children and six others.
Twenty-four passengers remained in hospital, including three children in Lausanne with critical injuries, but the other survivors were out of danger, Swiss officials said.
Swiss President Evelyn Widmer-Schlumpf and Belgian Prime Minister Elio Di Rupo, speaking at a news conference in the town of Sion near the crash site, paid tribute to the victims and the 200 rescue workers who pulled the injured from the wreckage after the bus rammed into a wall inside a tunnel on Tuesday night.
"When a drama like this happens, when we lose a child or have a child suffering in hospital, there are no words. It is important to console the families," Di Rupo said.
Twenty-one of the dead were Belgian nationals and seven were Dutch, according to Swiss officials. The Dutch Foreign Ministry said three Dutch children in the bus were injured. Most children aboard were aged about 12.
The man in charge of ambulances at the tunnel, Alain Rittiner, described a scene of horror.
"Access to the vehicle, which is quite high, was difficult and it was hard to remove the victims. Emotions ran high, and with so many children, it was an absolute horror," he told Swiss television.
A police photograph showed the bus had smashed into the side of a tunnel, with the front ripped open, broken glass and debris strewn on the road and rescue workers climbing in through side windows. It was later towed away from the scene.
About 200 police, firefighters, doctors and medics worked through the night at the scene, while 12 ambulances and eight helicopters took the injured to hospitals in the holiday region.
Belgium plans to hold a national day of mourning. A memorial mass was set for Thursday in the Swiss town of Sierre. Widmer-Schlumpf, a mother of three, said Switzerland was doing everything to support victims and their families.
Olivier Elsig, prosecutor for Valais canton (state), said that video surveillance images from the tunnel, where the speed limit is 100 kmh (62 mph), showed no other vehicle was involved in the accident and the road was dry and in "good condition".
"The bus did not appear to be travelling too fast," Elsig told the news conference. "I immediately ordered an autopsy of the deceased driver, which is under way at this very moment."
In Washington, a spokesman for the National Security Council said U.S. President Barack Obama "sends his deepest condolences to the victims and their families of last night's bus accident in Switzerland. The loss of so many young lives is especially heartbreaking".
It was the worst accident in Switzerland since 1982, when 39 German tourists were killed on a railway crossing after a train hit their bus.
The bus had travelled only 15-20 km from the Swiss ski resort of Val d'Anniviers before entering the tunnel. "The children were all wearing seat belts but the shock of the crash was violent," Elsig said.
There were three possible causes for the crash, he said: a technical problem; the driver may have become ill; or human error.
About 100 family members, who flew to Geneva from Belgium, were taken by buses to the Valais canton. Some began visiting injured children in Sion hospital, while others were being counseled by psychologists in crisis groups.
A mortuary was set up and bodies were being identified. So far, 22 of the 24 injured had been identified, officials said.
Gathering at primary schools in Belgium before boarding a military aircraft for Switzerland, some of the parents did not know whether their children were dead or alive.
"There's no news, simply no news," said one red-eyed father.
Of the 24 injured, three were in a coma. "Three children are potentially gravely injured and we hope that their condition stabilises, but we must remain guarded," Jean-Pierre Deslarzes, a doctor heading the rescue operation, told the news conference.
Most pupils were from the towns of Lommel and Heverlee in Belgium's Dutch-speaking Flanders region.
Children at St Lambertus school in Heverlee, a suburb of Leuven, were informed about the accident at an assembly before classes. Flowers were laid outside the Catholic school where eight children were still unaccounted for.
A teacher from St Lambertus were killed along with the bus's two drivers and three other adults.
Teachers covered boards outside the school and the school gates with their pupils' drawings - many rainbows, flowers and references to "Mister Frank", the sixth-grade teacher who died.
"I hope we will wake up soon and that this nightmare will be over," one pupil wrote.
The bus had not been driving for long after heading down from the resort and had only been on the valley highway towards the Swiss town of Sierre for 2 km (1.2 miles) when it hit the curb and veered head-on into an emergency siding in a tunnel.
The bus was owned by Belgian company Toptours, which Belgian Transport Minister Melchior Wathlelet said had a solid reputation. The driver had arrived at the resort a day before the trip, according to the rules, and the bus had passed a mechanic's test five months ago.
Of the 10 Dutch children on the bus, nine lived in Belgium and one in the Netherlands. There was also one German and a Polish person on the bus, the Belgian government said. (Additional reporting by Katharina Bart and Andrew Thompson in Zurich; Ben Deighton, Claire Davenport and Robert-Jan Bartunek in Brussels, Robin Bleeker in Sion and Denis Balibouse in Sierre; Geert de Clercq in Paris; Ivana Sekularac in Amsterdam; Writing by Stephanie Nebehay and Emma Thomasson; Editing by Mark Heinrich and Alessandra Rizzo)