A full-size U.S. school bus typically has 6 wheels.
The usual arrangement is two wheels at the front and four at the rear: the rear axle uses dual wheels on each side to support the bus’s substantial weight and passenger load. In everyday counting, that produces six wheels, although some descriptions count tires rather than wheel assemblies.
The number is typical, not universal. Wikipedia lists school buses more broadly as having four to six wheels, because smaller Type A and some other configurations can use fewer wheels. Full-size buses are generally Type C or Type D vehicles, and their heavier construction makes dual rear wheels common.
A related source of confusion is the phrase “full-size.” It refers to a large school-bus class, not necessarily to a particular engine position or body style. A full-size Type C may have a conventional hood, while a full-size Type D may have a flat front; both commonly use the six-wheel, two-axle arrangement.