 If you liked
Houkago Hokenshitsu
|
 ...then you might like
Angel Beats!: Track Zero
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When I started reading (and watching) Angel Beats, I immediately thought of "After School Nightmare". These series have a very similar setting, in which the characters are inside some kind of school that looks normal but is not. They have to fight, they have to find their "true self" and they disappear without anybody knowing what happens to them.
While Angel Beats has a lot of comedy, After School Nightmare has more suspense and dramatic moments as well as romance ... and in general ASN is one of the best mangas out there. Highly recommended.