- Esoteric Happy Ending: Even the happy endings are this at best.
- Toshiko and Naomi who hated each other and are as different as night and day end up Suddenly Sexuality and falling in love with each other.
- Mariko and Kaori end up Together in Death as mermaids. However, Kaori is still injured making her a very easy target.
- Really what can you expect from a manga whose premise is essentially teenagers murdering someone to Mind Rape their crush into loving them? Doesn't seem like the beginning of a healthy relationship.
- Fridge Brilliance: Hidemi and Nanae's tale reveals an explicit reason the mermaids turn feral. Looking back at the other two tales in which they turned feral, it's entirely possible that it was the case then too, as there isn't anything explicitly contradicting it. The Reveal in Yui and Saori's tale even hints that it could've been the case.
- Fridge Logic: Suppose that a girl named Hanako is in love with a boy named Tarou. Tarou had the surname Suzuki until two weeks ago, when his mother divorced and took custody of Tarou. His surname was thus legally changed to his mother's maiden name, Watanabe — although he's still listed in the school's records as Tarou Suzuki. If Hanako decides to win Tarou's love by eating a mermaid, does she eat a W or S mermaid?
- Jerkass Woobie: Yoshiko
- Les Yay: Implied as the reason Mariko willingly joins Kaori as a mermaid.
- Shouko got extremely jealous after Nanae got a boyfriend, and it's implied she became the first mermaid out of despair over Nanae seemingly abandoning her.
- Moral Event Horizon: Saya, after stealing Takaaki away from Shinobu.
- Nightmare Retardant: The last page of the first chapter kinda kills the sinister mood a little.
- Or makes it worse, since we're most likely seeing Yoshiko's mermaid right before Haruko kills and eats her.
- Paranoia Fuel: For the guys: Your high school sweetheart? Made you fall in love with her via mermaid cannibalism!
- For everybody: That old acquaintance you lost track of? She tried to do the above, failed, and now swims the corridors of your old high school!
- Another one for everybody: Your best friend? She plans on winning the guy she likes by setting you up as the victim of an elaborate plan that ends with killing you and eating your flesh.
- Tear Jerker: It's a little hard to read the story of Mariko and Kaori without getting a bit misty-eyed...
- The readers see the literal moment Saya succeeds in stealing Takaaki away from Shinobu.
YMMV / School Mermaid