If he didn't tell you by saying "You've got red on you", he's not worth your time.

For a second I thought you were the karate girl who's boyfriend got beat up by a feminine fury.

Crappy situation aside, get a period tracker app, and let it remind you 2 days before expected start. If I know I'll be away from home and/or doing activities, I've worn a light pad to prevent early accidents. Sometimes there's days I have no cramps but it still comes. Sorry about it being seen, I'd be mortified too!

Not all women are regular. Tracker apps don't work for many women, and even if they do work, they can be wrong on occasion.

Obviously women can be irregular, but the majority aren't. Especially if you're on BC. OP writes it out as if it was probably a normal time to start it, just not smack dab in the middle of a class. No where is it written "Today my period came 'EARLY'..."
Period tracker app still does help A LOT of women! Get into the habit of it sooner than later, and you won't be surprised as much.

There’s a gang of women waiting to beat the shit out of him for causing bleeding in your lady business.

They seem to like to congregate around dojos and make hasty conclusions to justify mob violence.

Aaaannnnnd here come the meta comments.

Oh shit! And don't you usually wear white in karate class? That sucks, OP. I'm going to assume it came earlier than you anticipated, otherwise you probably would have been more prepared?

Is your username just your name or some sort of pun or reference I'm not smart enough to get? I can't help but feel like I'm missing something here.

“Notice me senpai” or “I hope senpai notices” is an anime trope.

Was this before or after the gang of women drove your boyfriend off?

Sorry but if he was chuckling then he’s nothing but immature and I would find yourself a new crush !!!

ooooohhhhhh shit lol