Let them walk to school, too. Public transportation may be cheaper, especially when traveling alone. Inspect suspension and chassis parts for occasional misalignment. Bent wheels, axles, bad shocks, broken springs, etc. create engine drag and are unsafe at high traveling speeds. Remove vinyl tops - they cause air drag.
Rough surfaces disturb otherwise smooth air flow around a car's body.
Remove items that cause wind resistance, such as luggage racks. Remove excess weight from trunk or inside of car - extra tires, back seats, and unnecessary heavy parts. Extra weight reduces mileage, especially when driving up inclines. During cold weather watch for icicles frozen to car frame. UN-removed snow and ice cause tremendous wind resistance.