Emergency buses were called in to take children to school this morning after the normal operator closed without warning.
Tillingbourne Buses, which operated half the services in the Horsham area, stopped trading last night.
West Sussex County Council managed to get all the routes to schools covered and says stopgap services will continue until a replacement operator is found.
The bus routes to Forest School, Millais School and Tanbridge House are being operated by Aventa and Arriva.
A county council spokesman said no pupils had been left stranded.
Councillor David Dewdney said he was disappointed Tillingbourne had not given any indication it was going to close.
He said: "Had they done this we would have been prepared and had time to make better contingency plans.
"I would like to reassure bus users and parents of schoolchildren in the Horsham area that we are doing everything possible to make sure their services continue to run."
Tillingbourne operated 12 contracts costing the county council an annual £180,000. The contract was due to end in May.
A hotline has been set up for passengers to check whether their service is still running. Call 01243 777556 from 8.30am to 5pm and 07768 122048 at other times.
No one from Tillingbourne was available for comment.
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