Towards the end of the movie, when Ferris is saying goodbye to Sloan, he looks at her watch, and it reads 6:00, and he begins to run home. Then they show Rooney on a school bus full of students. At 6:00 in the evening????
The 6:00 bus is just a bus taking students to or from an after school event. Whoever thought that it was a slip-up obviously never went to a school with after-school activities.
I assumed that this scene had nothing to do with the timeline of the movie. It seemed like they were just showing what happened to Rooney after he left Ferris' house to bring closure to the plot. The time may or may not have had anything to do with the bus scene.