"Well, me and my friends are going to the pool today, so... obviously my swim diaper!" She lifts up her skirt, showing she's already wearing it. Whether that one would last until the pool, he wasn't sure. "Alright, but uhh... make sure to pack some extras, if you're already wearing it it might not last long enough..."
He decides it'd probably be more comfortable without clothes on, since it'd be at the pool, so he strips down before climbing in. He ends up moving slightly, as Amanda giggles and runs out to join her friends.
They hop on the bus; She never informed him that it'd be a school trip, but he didn't really care. Her bag contained her swimsuit alongside half a dozen spare swim diapers, although at the time he wasn't aware how she'd get her changes in, since he wasn't aware if she wanted him to be seen.
He could hear her talking to her friends, as she calmly sat on his waist. His face was right near her vulva, and he had to keep his thoughts calm in there to avoid alerting her with a boner.