A school trip is an easy way to get characters to let their hair down and get themselves into situations they wouldn't in school.
As often happens, Japan might as well be the size of a single city block. Julia and Rio are there because their school trip just happens to be to Hokkaido, at exactly the same time as Osaka High's. Nanba's there, even though he's a senior, because why not? Sano comes across his younger brother not once, but twice as the group wanders around town.
Some situations are a little too implausible. These kids have parents who would want to be confident their kids are being properly chaperoned, or so I'd hope. But Umeda's off to find a sex partner, and the students are taking an evening stroll in the red light district? Julia and Rio come visit Mizuki's group in the hotel (yes, both schools' groups are in the same hotel) and we see not a single adult corraling the girls.
I'm starting to warm to this series a bit more, although I still don't think it's exceptional. The characters are fairly likeable, their interactions are amusing, and the art and story are decent. My prediction is that I'll run through it once and then it'll go into the box for the used bookstore.