Haha, you said it, exactly!
A super embarrassing, top-secret, secret! Even on a non-PE day, you weren't ever guaranteed that no-one would see them. What if the waistband showed? (My friend Jason) What if someone looked under the cubicle door? (Me!) What if you accidentally forgot to do up your flies? (my friend with the dinosaur pants) Or worst of all, what if someone pulled your trousers down / off (me again!) or gave you a wedgie?!! 
Yes, I'm sure loads of other boys had exactly the same 'hierarchy' of pants at that age. We can't be the only ones! It's fun to think as well that a fair lot of the 'cool kids' who wore boxers on PE days probably also wore briefs on other days but we never knew at the time. It was always brilliant when someone you thought only wore boxers slipped up and occasionaly turned up in briefs on a PE day! The illusion that they'd switched for good was shattered and you could never be sure they weren't wearing briefs the rest of the time after that.
And yes, a lot of the cartoon / picture designs they put on pants I always thought were really strange choices, especially the ones with a single picture / emblem / word right over your...
drawing attention right to it! The days of the week -- as if looking at your pants was a great way to remember them! And anyone with 'Cool Dude' written on his underwear, even if they were boxers, was clearly not 'cool' at all! ( Well, I don't know, maybe you were and just styled it out
)No, I don't know why she stopped getting me white or cartoon, I don't remember ever asking her to. Maybe she just sensed. In fact I never spoke about my underwear, full stop. It was always far too embarrassing a subject to bring up with anyone, even my Mum. I remember from about the age of eleven or twelve really wanting her to start getting me boxers, or at least only dark, plain coloured pants but I was never brave enough to say so! I could have worn boxers like pretty much everyone else if I'd only been able to ask but weirdly the thought of talking about my undies with my Mum was so excruciating, I just put up with staying in pants until I left school
Oh for sure! If she'd drawn attention to me when I was just in my pants, laughing about me being 'half naked' it would have been devastating! I never undressed down to just my pants anyway, so they would have at least been half covered by my shirt. Some boys in my class got changed like that and I always thought it really odd. But no, I didn't get teased when she said it and I remember being very relieved no-one did. I think, like me, everyone just found it a kind of ridiculous thing to say. Phew!
Oh no! Those Mickey Mouse pants sound truly awful to be 'caught short' in
There's also something about pants in general when, as you say, they're 'pristine white'. Somehow, for whatever reason, the brighter, purer white they are the more embarrassing they are. 'I hadn't put on the ‘better’ pants that morning- if there are such a thing' -- haha, I know what you mean. There definitely are 'better' pants than others, and those Mickey Mouse ones are definitely not it!
Yes, I did slip up and get caught short like you a few times over the years. I think we all do at some point or other. I remember a boy in about third year at senior school getting laughed at for wearing Winnie the Pooh / Tigger boxers; or maybe it was Scooby-Doo or something. But what was so strange is it was one of the 'tough' 'cool' lads and they were boxers he was wearing. I guess if the design is suitably childish, it doesn't matter if they're briefs or boxers, or who wears them and why, even if they wear them ironically -- you're gonna get the piss ripped out of you
How old were you the last time you slipped up, and what were you wearing?
I think for me it was probably a bright, vivid turquoise pair I used to wear in about third or fourth year. I couldn't get changed fast enough that day
Fortunately no-one commented so I got off lightly, or maybe no-one saw.Do keep up sharing your stories! I love hearing them.
Bobby