Wouldn't Jeanie recognize her own school principal?

Amazingly, human recognition can be situation-specific and poor at times.

There's an old Columbo episode (might've been the first), where a mistress of a doctor is worried, because she's supposed to 'play-act' the doctor's wife on an airplane full of strangers, and she's worried they would recognize that the mistress is not the wife.

The doctor simply says, 'people see what they expect to see'.

Now, it was just a TV show, and this is obviousl not the full truth, but it can be surprisingly true sometimes. If you're not expecting to see a celebrity, and someone is dressed in a janitor's uniform, you basically 'see' a janitor, even if it's actually someone famous (more or less with somewhat hidden face, presumably).

There has been japanese idols that have 'infiltrated' a school secretly (with permission from the higher-ups), and taken big risks by dressing as an Assistant Director or some such thing, and assisted with setting up things, and no one recognized them, because they didn't expect actual idols to be dressed as basically janitorlike clothing, doing typical AD things.

So Jeanie expected to see Ferris, but ended up seeing 'someone that is not Ferris, and says 'Haa!'', so basically she 'saw' a scary burglar that might harm her, and her fear kept that illusion going. It's plausible.