That's probably because he wasn't sick, he was skipping school.
7 seconds sound clip from the Ferris Bueller's Day Off movie soundboard.
You can hear this line at 00:09:36 in the Blu-ray version of the movie.
Quote context
[...]
- I have no reservation, whatsoever, about holding him back another year.
- So far this semester he has been absent nine times.
- Nine times? I don't remember him being sick nine times.
- That's probably because he wasn't sick, he was skipping school.
- Wake up and smell the coffee, Mrs. Bueller.
- It's a fool's paradise. He is just leading you down the primrose path.
- I've got it right here in front of me. He has missed nine days.
- I asked for a car, I got a computer. How's that for being born under a bad sign?
[...]