But my car has a BMW-factory top and BMW-factory rollbars. Why isn't this good enough?
At a drivers' school, you will encounter speeds that are far higher than those encountered on the street. Recall from high school physics that the energy of your vehicle is proportional to the square of its speed (e.g., your car's kinetic energy at 120 MPH is four times the energy at 60 MPH). This energy must be absorbed by your vehicle's structure in the event of an impact. Structures that may provide adequate protection at speeds normally encountered in day-to-day driving may not be adequate at the higher speeds involved in drivers' schools.
Because we cannot test and therefore ensure the integrity of these structures, we have elected to err on the side of caution and prohibit cars without structural A- and C-pillars from our drivers' schools.