A five-week-old school bus drivers' strike that had affected 27,000 students has ended with the signing of a new contract by Mayor Raymond Flynn.Striking bus drivers, members of Local 8751 of the United Steel Workers Union, ratified the new four-year pact Saturday night after the School Committee voted 8 to 1 to accept the deal. The union was notified Sunday afternoon Flynn had signed the contract. Although he missed the union-imposed deadline for signing, union leaders said they would be back on the job Monday. The contract does not provide the sought-after pay raise for the drivers, but it guarantees that whatever company holds the contract to manage the bus fleet will hire workers from the present work force. The union was concerned the city's school bus system would be revamped when the current contract with In-City Boston Management expires in June. The drivers had been seeking a 14 percent increase.