Because you know I always want a cup of tea

The reason was that, if you get pantsed, and you weren't in on the joke / it wasn't planned, that's a massive breakdown in respect and discipline, and you have to make an example of that kid (generally by wrestling them, and in serious cases, taking away candy privileges). But getting pranked is still a bad look, and makes it seem cool to rebel against your authority.

However, if you get pantsed, and you are in on the joke, everyone has a good laugh, including you, and no one was actually rebelling. It both makes you look like a cool authority figure and makes the person doing it look like they're the sort of person in cahoots with counselors. Then, if there's a behavioral issue, you can have that quiet conversation later, away from an audience.

And since those underwear are so culturally specific as punchlines in a pantsing gag that the only plausible reason to be wearing them is if you're in on a slapstick act, you can retroactively Shanghai any would-be prankster into looking like they did it with your consent and planning, which not only keeps you from indignity, it makes sure that they're rewarded by laughter and attention for looking like they're cooperating with the staff, encouraging that in the future and bringing them in from the outside of the social-reward structure you're trying to set up, where it's cool too be wacky but responsible.

That preparation effort paid off maybe four times across three years, but it was completely worth it.