Other mistake: Ferris wants to skip school and who can blame him? In the boring lesson early in the movie, supposedly at the start of the day, look at the student's watches. They all read 6.25. (00:11:10)
Other mistake: In the scene where Cameron is sinking in the pool you can clearly see some kind of weight belt or something around his waist, which isn't there when he is out of the pool.
Answer: Sloane knew it was a hoax. Ferris would not have done something like that without her knowing. Rooney pretty much had figured it out, but could never prove anything. Once he was discovered inside the Bueller house (illegally) by Ferris' sister, anything he did know he would have to keep to himself.
raywest ★
Sloane starts to put her jacket on the moment that she sees the school nurse walk into the classroom, suggesting that Sloane already knew she would be called out, and she smiles to her classmates.