Hohzuki Island is a relatively short manga by Kei Sanbe, the guy who also wrote Boku dake ga Inai Machi - which just got an anime this season, but I haven't tried it yet - and Cradle of Monsters, a mediocre zombie-manga about kids trying to survive on a boat. And well, there's some consistency there, because Hohzuki Island is a mediocre survival/horror/mystery manga too.
First off, I have to say that for the most part the building of suspense was well done. The adults are suspicious, there are mysterious disappearances, a ghost roaming around, and an interesting conflict where you feel like anyone could die at any given moment. It makes you want to finish the manga in one go (which is possible since it's only 4 volumes). Having the perspective of the kids is also crucial here, and is important for the ending too.
Another good point is that there are few questions left unanswered. Shorter series often gloss over certain key points in the plot, but here everything is explained. Although some explanations were complete bollocks, we got something to say the least.
But then we get to the ending, which was even worse than the classic "it was all just a dream". A ton of reveals are handled really badly before all this, but at the ending we get the biggest mess. Things that were explicitly shown before are just ignored, a ton of impossible things happen, and it was completely ruined by a certain overused deus ex machina. It also didn't fit with the tone the story had before it, and felt rushed. For mysteries the most important thing is the ending imo, so I gave the story a low score overall even though the suspense was good.
When taking a look at the characters, I can only be very disappointed. They're all so one-dimensional it isn't even funny. What do we know about them? The teachers: one is evil, one is semi-evil, another is an evil pervert, the fourth we know nothing about. The kids: one likes eating, one is blind, one is smart, one is mute, and one is the main character. Defining charactes with one trait like this is lazy imo. Adding a "super-intelligent" kid to the bunch is only a recipe for lazy storytelling ("I just know because I'm smart").
The artstyle is nice enough, yet there is one thing I have a huge problem with: Yukino, the female teacher. Her hip-to-waist ratio varies from 1.5 to 2.0. So first off: decide on one ratio, and secondly: make it a ratio that's actually possible. Also, there's an abundance of tits and asses. Not necessarily a bad thing, but not for everyone's tastes so be warned.
Overall: the concept is solid, and there's enough suspense to keep most people invested. However, weak characterization and a bad ending make this manga go on the long list of mediocre and forgettable works.