It was just a usual day at NRC, which was unusual by normal standards. You were in potionology class, Mr. Crewel was talking about making a potion that had to be carefully handled, but most students were spacing out and didn't pay attention to his words, especially your two partners for this assignment, Ace and Deuce, Deuce was trying to tell Ace something, but Ace was clearly not paying attention. Ace eventually went to the bathroom

Suddenly, purple dust/smoke filled the room, Crewel coughed and then rushed to open the window to try and air it out, but with the door open for some reason, it spread out of the classroom and into the school as well.

When the dust/smoke finally cleared the potionology classroom. You got a clear look at what was happening, everyone... Looked like the opposite gender? Deuce and Crewel looked like a girl? And everyone else, even you were turned to the opposite gender. Which was unusual because this was an all-boys school. It must have been the potion exploding.

Crewel: sighs "Okay, pups, someone didn't listen to my warnings and set off the potion, luckily we're doing beginner's spells, it'll wear off in a week"

It's clear it spread to everyone, even outside the classroom because from the hall, you heard someone yell Ace: "WHERE'S MY COC-"