Ferris, speaking here about his "I'm sick" ruse, wastes no time in establishing that his folks are easily duped. He also wastes no time in breaking the fourth wall and making the audience co-conspirators in his schemes to thwart authority. He has no doubt that you'll be on his side, Shmooper.
FERRIS: Incredible. One of the worst performances of my career, and they never doubted it for a second.
Just in case it wasn't clear that his parents are oblivious.
ROONEY: Last thing I need at this point in my career is 1,500 Ferris Bueller disciples running around these halls. He jeopardizes my ability to effectively govern this student body.
GRACE: Well, he makes you look like an ass is what he does, Ed.
Grace keeps it real, dude. You go, Grace.
ROONEY: Once again, let me tell you how deeply saddened I am by your loss. I had a grandmother once. Two, actually. Man that is born a woman hath but a short time to live and is full of misery. He cometh up, and is cut down like a flower. He flee-eth, as if it were a shadow.
Sloane rolls her eyes.
ROONEY: And never continue-eth in one stay.
Sloane stares at the empty driveway uncomfortably.
ROONEY: Oh dear. Between grief and nothing, I'll take grief.
Continue-eth? What the heck is Rooney even saying here? Nothing of substance, that's what. Fearing that he's insulted Sloane's dad, Rooney is trying to suck up to Sloane herself with this thoroughly warped—and utterly cringe-worthy—attempt at faux Shakespearean verse. Sloane's not impressed.
ROONEY: Grace, Ferris Bueller's behind this. There is no doubt in my mind, and now he's got Sloane Peterson involved in this thing.
GRACE: Her grandmother, too.
Aside from her encounter with Jeanie, where Grace calls the teen on her mean-spirited muckraking, Grace is basically portrayed as an imbecile.
MAÎTRE D': I appreciate your understanding.
FERRIS: Don't think twice. It's understanding that makes it possible for people like us to tolerate a person like yourself.
Whoa—Ferris just straight-up tricked the Chez Quis maître d'. And the fact that he can not only steal another party's reservation from under the suspicious maître d's nose, but also get a sycophantic apology from him, speaks volumes about the role of adult authority figures in the film.
Rooney and the Pizza Man watch the Cubs baseball game. ROONEY: What's the score?
PIZZA MAN: Nothing, nothing.
ROONEY: Who's winning?
PIZZA MAN: (beat)
PIZZA MAN: The Bears.
If there's any doubt that Rooney's a doofus, it goes out the window here. The fact that this exchange comes right after Rooney mistakes a young woman for Ferris Bueller doesn't help his case either.