A horror manga I've recently read was School Ningyo , also known as School Mermaids. The plot revolves around a school where an urban legend says that at night "mermaids" will appear in the school and if you managed to kill one wearing the first (or last?) initial of your crush and then eat it while thinking of that person, then that person will fall in love with you. It's very short though (it's about two to three volumes long). But if you are bored, it's a good read and a little creepy.
There's also Gakkou Gurashi, or School-Live, which can best be described as a moe slice of life set during a zombie outbreak. It's still ongoing, and it also has an anime adaptation.
The Kurosagi Corpse Delivery Service is also a good one.