I highly recommend Kareshi Kanojo no Jijou. I read this at my local library at that age and loved it. It contains a lot of good life lessons as well.

"We can not resort to simplistic or extreme solutions which substitute myths for common sense." ― Jimmy Carter

Here are some that are fairly recent (so still in print) and aren't really directed at any specific audience besides "young-ish."

Still ongoing, but most volumes are fairly self-contained

A Terrified Teacher at Ghoul School- This is actually my favorite manga. It also has some cool yokai facts

Heaven's Design Team- An interesting look into why animals have evolved the way that they have (the answer is usually osteoporosis). It has an anime

Cells At Work!- This one is kind of famous already, but it's a good starter manga. Maybe get the spinoffs if you've already got them at the library.

Dragon Goes House Hunting- Recently got an anime adaptation. It's got interesting world building

Sorry For My Familiar- Not quite as self-contained and still ongoing. If it makes it more accessible, I think you only really need to buy the first five volumes or so to complete the first arc, and if the students want to read more, then see about buying the rest.

Magus of the Library - Also one of my favorites. Again, this is also one that isn't as self-contained, but it is absolutely worth it reading, especially to encourage kids to learn and read. I as an adult reading it felt that. The art is beautiful and the world-building is very engaging.

Completed (ish):

Ore-sama Teacher- From the same author as Monthly Girls Nozaki! It's completed at 29 volumes.

Hinamatsuri- Oh, how quickly we forget one of the funniest series of the decade. It's completed at 19 volumes, but so far only 12 have been officially released

Sakamoto Desu ga? - Also one that had a popular anime and then was quickly forgotten. 4 volumes.

Skull-face Bookseller Honda-san- A comedic bibliographical take on working at a bookstore in Japan. 4 volumes

+Anima- I'm gonna be honest, this one's hella old and I literally only have ever seen school libraries carry copies of this. I think they just kind of grow from school library shelves. Good luck finding copies if they don't. It's 10 volumes and I kind of wish there were more, but it's from the time where stories had a planned beginning, middle, and end.