This review is for the original series and the the revised versions that just released. I read these when they first came out and I truly enjoyed them (there are 3 volumes in both the old and new iterations).
As a librarian who is always looking for new books to add to a middle school library, I'm always hoping to find appropriate manga which is a VERY hard job with a ton of research involved, so I always try to leave notes for others in the same profession. In terms of sexual content, nudity, or even revealing clothes or disrespectful handling of women characters like sexual objects, this series is free of all of it. It's pristine and completely appropriate. This is hard to find even from western-based mangakas and I applaud Queenie Chan for that immensely.
However! I would be very careful giving this to younger middle grades. This is more appropriate for 8th grade on; it's rated 'teens' which is pretty much always 13+ in the manga rating world. The reason for this rating is because of the intense horror elements. Both the original series and the updated series rely on the supernatural as the root cause of the girls who go missing from the school.
Spoilers below:
Characters have unsettling dreams, and in the new version H. P. Lovecraft's fictional works are referenced, including the 'Necronomicon'. The 'Fairy King', which was an Aborignal mythological spirit called a Quinkin in the original, has been changed into a dark Lovecraftian figure from another world and its depiction is honestly more unsettling and more prominent in the story. I honestly did not like this change, as the Quinkins fit the Australian bush theme very well and taught me something new; I also have no appreciation for Lovecraft or his works and was disappointed by this turn. But yeah, if you get the original books, you'll have a story that revolves around spiritual beings called Quinkins as the antagonists—they're depicted as evil spirits that lure children into the bush to steal their souls. (Technically these are the Imjim varients of the Quinkin which are evil, though the image of a depicted one is a Timara, one of the good types of Quinkin.) Oh, and in both versions, the girls try to have a seance/ouija type session at the beginning to try to contact the missing girls from the past; this causes one girl to become possessed/lured away later.
There is also a very prominent collection of macabre images. Skeletons of women in dresses are drawn in mental imagery and in chapter pages; a girl's dead body appears in a small watering hole, though not decayed; paintings depict girls chasing others with knives, which is eventually revealed to be an omen because it later happens to a pair of sisters; blood is a prominent image from weeping sap trees to scenes of self defense in which a character uses an axe to defend herself and her sister from said girls with knives. Nothing is shown of the actual scene using it, but we see her later splattered with blood and the axe has blood all over it. The sister mentions how the other girls will die from blood loss from the wounds, being horrified that her sister essentially killed the others, even in self-defense.
Ghosts/beings from the other dimension/Quinkins possessing girls' forms in the originals show up in the 2nd and 3rd volumes and roam the school in search of other girls to capture/lure away/steal the souls of. Even as an adult, the eerie images made me feel like I might have nightmares after reading them just before bed, haha!
Overall, the series is very engaging thriller/horror mystery. I do prefer the originals, but the updated art was a treat in the newer, revised series. Apparently there will also be a continuation following the revised series.