Kumo Desu Ga, Nani Ka?

A brief description of the Kumo Desu Ga, Nani Ka? manga:

A mystical case leads to the death of a whole class of Japanese schoolchildren. Instead of the afterlife, schoolchildren are transported to a parallel world, gaining a new chance at life. The main character, the most unpopular girl in the class, gets a special “prize” from life. She becomes a spider in a cave, where it is not so easy to survive. But can she survive in a dungeon full of dangers, relying only on luck and her own mind?