Diego drew a scale drawing of the elementary school. The scale he used was 9 millimeters : 4 meters. In the drawing, the schoolyard is 324 millimeters long. What is the length of the actual schoolyard?
If 9 millimeters on Diego's drawing represents 4 meters, then each millimeter on the drawing corresponds to 4/9 = <<4/9=0.4444>>0.4444 meters in reality.
Thus, the length of the actual schoolyard is 324 millimeters * 0.4444 meters/millimeter = 143.9996 meters.
Since we cannot have a fraction of a meter, the length of the actual schoolyard is 144 meters. Answer: \boxed{144}.