A school bus caught fire shortly before 7 a.m. Wednesday near 109th Street and Florist Avenue, close to Timmerman Airport, after a neighbor alerted the driver that the vehicle was smoking, officials said. The driver evacuated the students and moved them away from the bus before calling 911, according to a letter from the principal of William George Bruce School. Witnesses said they heard a loud boom shortly afterward, followed by flames. A replacement bus arrived, and students were taken to school, where staff welcomed them. The principal said the incident may have been upsetting and invited families to contact the school office for support from counselors, nurses, psychologists or social workers. No one was injured or taken to a hospital. Officials said the fire appeared to have started near a rear tire. They had not confirmed how many people were aboard; the school principal’s letter said students were on the bus.