Sixteen other pupils were injured - one very seriously - in the accident, which happened when the vehicle, with 32 pupils on board, left the narrow B7016 on a sharp bend on its way to Biggar High School, Strathclyde.
Last night there were renewed calls from motoring organisations and the Labour Party for the fitting of seatbelts in coaches and buses to be made compulsory.
Children were sprayed with shards of glass as the bus windows smashed and Francis Scorgie, 15, of Carnwath, was hurled through a side window to his death. Tony Cousins, 13, also of Carnwath, who was seriously injured, was taken to Glasgow Southern General Hospital where his condition was 'very serious' last night.
Only one of the other 15 injured teenagers was detained in hospital overnight for observation. The two drivers involved were released from hospital after treatment for minor injuries.
Henry Weekes, 64, of Carstairs Junction, said that he saw Francis land on the road. 'He was flung out of the window. The children were very shocked and some of the girls were crying. Lots had cuts.'
The crash has renewed calls for safety belts in coaches to be fitted compulsorily. However, the bus and coach industry is sceptical, arguing that belts would be expensive and technically difficult to install. Some other European countries are also opposed to their introduction.
James Hood, Labour MP for Clydesdale and chairman of the Commons select committee on European legislation, said the accident would be raised when MPs met to consider road safety next week.
John McFall, Labour Scottish transport spokesman, said coach-builders should be required by law to fit new vehicles with seatbelts. All coaches used for transporting children should be fitted with belts immediately.
Derek Prentice, assistant director of the Consumers' Association, said: 'All previous research suggests that seatbelts in coaches and minibuses save lives. We urge the Government to act unilaterally from its EU partners and insist all British-registered coaches are fitted with these life-saving devices.'
Mr McFall, MP for Dumbarton, added: 'It is quite perverse that we insist on seatbelts on planes and the back seats of cars, and yet the Government are not prepared to insist on them in coaches carrying scores of children.'
The Department of Transport said that seatbelt regulation was a matter for European Union law. Britain could act alone but legislation would be difficult to enforce.
Thirteen children died last November when their school minibus ploughed into the back of a motorway maintenance vehicle on the M40 near Warwick. Eight days earlier, 10 people were killed and 36 injured when a coach carrying American and Canadian tourists crashed through a safety barrier on the M2 in Kent after colliding with a van.Reuse content