 If you liked
Houkago Hokenshitsu
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 ...then you might like
Torikago Gakkyuu
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Both have a main protagonist stuck on a strange high school where a teacher gives him "missions" to save someone. In ASN the protagonist enters a sort of another dimension that is like a DreamWorld to help other characters until they wake up and he has a time limit. In TG the protagonist does not enter the other Dimension but he also has a time limit to change events in this world so that the other world doesn't get unbalanced. ASN has a continuous plot while TG is more comedic, episodic and random.