Hours later, with most still in the dark on the fate of their children, they were boarding a government Airbus for Switzerland to words of comfort from King Albert II and Queen Paola.
"They're sitting there, inside, without knowing," said parish priest Dirk De Gendt as the town of Heverlee tried to absorb the full horror of the overnight tragedy in a tunnel under the Swiss Alps.
"Parents who know their child is alive are relieved, but for the others it's terrible," he added.
After being informed of the crash by phone, parents began arriving at the school at 7am, some returning home to fetch their things before heading to the military airport.
A total of 24 pupils, aged 11 and 12, along with their teacher and a teaching assistant from the tiny Catholic school in the Leuven suburbs, were aboard the coach when it swerved and smashed at high speed into a concrete wall in the tunnel with 52 people aboard, most of them children.
Both the adults were killed, said De Gendt, who rushed to the Sint-Lambertus school in the early hours to help both the relatives and the pupils.
The teacher was "a very dynamic" man, according to a member of the school board who said "a great sorrow" hung over the establishment. A child's painting near the gates said "for Franck", the dead teacher.
Of the 24 pupils, "16 suffered different injuries, some broken arms or legs, but they are alive," said De Gendt.
But the school had "no news" of the remaining eight, added the priest, who knew the children well as he had been preparing them to take communion.
The high number of injuries from Sint-Lambertus appeared to imply bad news for students from another school in the north-eastern town of Lommel near the Dutch border.
Belgian television said the Lommel students, nine of them Dutch, had been sitting at the mangled front of the bus.
TV images showed tearful parents running into the Lommel school building in hope of news as strained family and friends huddled outside to wait.
And as families gathered for the flight to Switzerland later in the day, many remained in the dark over the fate of their children.
"We still don't know, we have no names, I don't know," said the godmother of one of the 46 children aboard the Belgian coach when the accident occurred.
"This is a tragic day for all of Belgium," said Belgian Prime Minister Elio Di Rupo, who was also flying to Switzerland but first joined the Belgian royal couple to comfort relatives at the military airport outside Brussels.
The children were returning from a skiing holiday organised by the Flemish Catholic school system.
Belgian Archbishop Andre-Joseph Leonard met the parents in the morning and said they had "felt something that resembled the cry of Christ on the cross 'My God, why did you abandon me?'"
With the entire nation in shock, the prime minister was to chair a restricted cabinet meeting and hold a news conference before leaving for Switzerland.
"Belgium awoke in mourning," the RTL-TV1 network said as it opened its noon newscast.
The European Parliament held a minute of silence at noon as the presidents of the European Union and European Commission, Herman Van Rompuy and Jose Manuel Barroso, extended their condolences over the tragedy.