A fart that serious wouldn't seem real, and that goes double if it appears that you unmuted specifically to make the noise. Just tell people it was a soundboard.
You'd rather be the person who decided to randomly unmute to share your actual factual filthy self? At least a soundboard is just a poorly judged joke (that actually landed, if the teacher laughed). Or you can be a fart pervert.
I'd always heard if you're presenting in front of a crowd and rip a fart, say something to the effect of "I was hoping to finish with a bang, not start with one." But I'm not sure how relevant that is for OP's situation.