A few of these are from shonen magazines, and a few have high school age characters in them, but I don't think they break your requirements in spirit.

Boku no Mura no Hanashi — As far as I can tell, there's only one volume available in English, and it's only available online. It's a fictionalized retelling of the Sanrizuka Struggle which is an important chapter in the history of the modern Japanese Left.

Yokohama Kaidashi Kikou — This is a chef d'oeuvre of gynoid fiction and the most relaxing depiction of the middle of mankind's extinction I know of. I strongly recommend it. It's about a robot girl called Alpha who runs a quiet countryside café near a Yokohama which has been submerged underwater. Alpha takes photos, she has human friends and a robot friend, she plays music on her moon guitar and awaits her owner's return.

Non Non Biyori — This is about the lives of four kids in a small farming community in the middle of nowhere: a first-grader, a new kid, and two sisters; and there's also a fifth kid but he never talks. Sometimes the chapters focus instead on the grown-ups around these kids. The anime adaptation of Non Non Biyori is great as well and if you've seen it you won't mind re-experiencing it in manga form.

Akane-banashi — The main character Akane's dad failed a rakugo test and basically gave up on his dreams of becoming a rakugoka after that, and so Akane is sort of trying to avenge him by becoming a rakugoka herself, and she dedicates her all to it and kicks ass. It's top tier.

A Bride's Story — It takes place in 19th century Central Asia. I just think that's cool.

My Journey to Her — Autobiographical manga by a Japanese trans woman. I guess it's been translated into English since I last read it myself, judging from the fact that it suddenly has an English title on Anilist, when it used to be called "Boku ga Watashi ni Naru Tame ni". A positive note is that since this manga was first published, some of the information in it has become dated due to progress in trans rights in Japan.

Yotsuba&! — This one is an absolute classic. The main character is a five year old girl adopted from a non-specific foreign country by a single dad, and the manga is basically just this kid's random adventures. It's the number one most recommended manga for learning Japanese. I guess you can say that Yotsuba&! is kind of like urban Non Non Biyori in a sense.

Aria — This one pairs famously well with Yokohama Kaidashi Kikou because it's in a similar vein of relaxing manga depicting a future which, for all its new technology, is still pretty "rustic". Aria is about a terraformed and renamed Mars far off into the future, where a 1:1 replica of Venice has been built, and the main characters are all gondolier tour guides. It's really imaginative and optimistic. Note however that Aria was originally called Aqua.