From Miista MangaFox:

"Sensei, is this a boy or a girl?"
"Do you want to know?"

Ichijo Mashiro and fellow classmates are enlisted in a mysterious weekly class that takes the form of a shared nightmare. Like most teenagers, Mashiro has a secret.. he's neither fully male nor female!

So far, Mashiro's been able to live life as a boy, but all this changes when he's informed of a new class he 'must' partake in order to graduate from his elite preparatory school. To pass, he must find the key and the one way to find it is to enter into a nightmare world, where his body and soul are put at the mercy of the worst kind of enemies -- other school students, who are battling to find the key.

From renowned author Mizushiro Setona, comes a riveting epic mystery that lies between the students, that will have you engrossed from one page to the next. Happiness, sadness, blood and tears will all be shown, but which one leads to an ending? Take the next step into the world of After School Nightmare!

From ShoujoMagic:
A collection of short shounen-ai stories by Mizushiro Setona, all connected by the theme of plantlife.
[STORY 01] (English Title) Breakfast (Japanese Title) ブレックファスト (Romaji Title) Burekkufasuto: The tale of a boy who displays the symptoms of man-eating syndrome.
[STORY 02] (English Title) The Tale of the Ring (Japanese Title) 指輪物語 (Romaji Title) Yubiwa Monogatari: The tale of two boys who are dating each other and mutually hiding the fact that both are dating their teacher, who frets because a ring has been lost.
[STORY 03] (Japanese Title) Honey β: The tale of a flower that blooms by inhaling memories. But even if our memories are taken from us, some precious things are never forgotten. "I don't fear losing my memory if I can feel this way again tomorrow..."
[STORY 04] (English Title) Undying Illness (Japanese Title) フジノヤマイ (Romaji Title) Fuji no Yamai See "Mister Mermaid," story 2.
[STORY 05] (English Title) Aconite (Japanese Title) トリカブト (Romaji Title) Torikabuto

From Tokyopop:
It's senior year, and Rica is seriously stressed out. She's got more homework, exams, and papers than she can handle, and was recently dumped by her boyfriend. In other words: Life sucks! One day, she's chatting online when she meets three classmates who hate school as much as she does. Together, they hatch a plan to blow up the school when no one is present. The countdown is on for X-Day ...

Featured in this manga is a side story called The Last Supper.