This may be the best description of bowel desperation I've ever read. It more than lived up to what I imagined when I read the challenge (I considered responding myself, but couldn't get started). My favorite part is "her hands clenched into tight fists at her sides, mimicking her rear."
I like how you structured this story, with her on the cusp of a public accident, then a flashback of how it came to be, then the accident happening in full and her getting away with it. I was honestly in suspense. My only major complaint with this story is that I wish there were more. I would like to read about her getting to the bathroom and cleaning up after she left the room. It would be especially neat if she ran into one of her friends, who was also out of class, and had to get past them before they smelled or noticed anything wrong.

Actually, more dialog throughout might help. Everything before fifth period is described rather briskly, and it might help if we had fleshed-out scenes of her meeting friends, debating about lunch, getting startled by the bell etc. One minor complaint: I can tell that you intentionally wrote the "present" scenes in present tense and the past scenes in past tense, but you slipped up a few times and used the wrong tenses.

Regardless, this is a nice non-predictable desperation story that entertained me, didn't offend or disgust me, and is written in a way that let me clearly picture what was going on, inside and out (and I do enjoy picturing sweet petite Tsukasa with a big brown banana weighing down her panties).