Can't they ask their parents for a quick trip to the sea? They would only need one of his or her parents for it. That being said, they should be able to go on their own on a bus and/or train. Middle schoolers should already be capable of it, unless the route is awfully complicated. Especially An who goes to a bigger school in a more significant town, further away. This is the time of smartphones, making everything much simpler, anyway.
Hmm, let's make some guesses.
Azeno appears to be based in the central mountains of Kyoto, let's say somewhere between Kameoka and Ayabe cities. If we assume that their morning bus trips to their school also lets them get to the train station (where bus hubs often are in Japan) then we might be able to say that one of the bigger towns along the San'in main line is where their bus transfers take place. Also important to note that Shunpei is 40 minutes away from his school by bus, so we can have a rough guess of distance. If we can trust the map on Chapter 10, Azeno is on the west side of the railway, and it's also where the railway is crossed by the expressway, meaning we can narrow it down to being based around the area of Miwacho, in the mountains south of Fukuchiyama. A travel time of 40 minutes is enough to go from Miwacho to Nantan, but a bus makes extra stops so we'll have to cut short, then let's say Shunpei's school is in Kyotamba town, it's also inside the rough area thats shown in Chapter 14 as well. Now we've roughly zero'd in on the area they may be based in.
If we assume that they take a bus in the different direction (An's school is shown to be to the south) and go north instead, they may choose to go to Fukuchiyama to take the train to the beach. If they're going to the nearest beach (Torii beach), that's a 1:30 minute train ride with 1 transfer in Maizuru for one direction + 30 minutes bus since Fukuchiyama is closer to Miwacho + 15 minute leeway for transferring to the train, 4:30 total travel time. Tottori is also a choice, but that's a full 3 hours train ride one way.
With this we can say that it might be too much of a hassle for them to ask for a quick trip, because it would likely take the whole day, and they're out in the countryside where there is a lot less people to see you should something happen.
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