When a school bus driver calls in sick, someone has to pick up their route in a hurry. Students can’t be left stranded. If a school district is short of backup drivers, other drivers end up running two routes, delaying pick-ups. Crowded buses and student tardiness is the result. This is not unusual for school transportation departments across the nation. It’s the norm. At Parkside Middle School in Manassas, Va., about three of the school’s 31 drivers can be absent on…