<human>: This happened about 15 years ago, but I still feel the shame.

When I was in 7th grade, my school had this class party thing where our class would all go out bowling, then swimming and then go back to school. 
For swimming, we were out on the beach. I live in a very hot climate, our teachers made us drink a lot of water. After only drinking one bottle, the teacher tells me to go get more and drink that. I take it and drink it, no biggie.

Here comes the FU.
The bus comes (late), and through that entire time I couldn't go to the bathroom. It was too far away, and I had to hold it.
I sit alone, because the bus was really roomy. It was a full 40 minutes before we got to school.

At 10 minutes through the bus-ride, it feels like something is stabbing me in the pelvis. I try to not pressure it, but it's close to exploding. We are on the highway, so getting off was not an option.
I take all my swim clothes out of the bag and put them in the bag, I cover myself with my towel. I take off my pants slightly, and slot dick into the bag. The bag was definitely not made for liquids, so it has a high chance of leaking. I piss into it and close the bag, in my haste, I spilled piss everywhere. 
I toss the bag into my schoolbag, and wash out the clothes I am wearing using water, which was very messy. 

Luckily, nobody in the class saw. Which was likely the only upside. But my smaller bag leaked all over my schoolbag, so it had piss in it. My swim clothes did too.

Give me a shortened version of the text
<bot>: Had to pee. Ended up using a towel covering me while I piss in a bag and spilled some. It got all over my schoolbag too.