Original novel by Yamada Yuusuke and illustrated by Sanbe Kei.
There is also a Japanese drama adaptation.
Description from the drama's wiki: When an armed group seizes control of an elite high school, a group of classmates are forced to find 2,000 puzzle pieces hidden throughout the school within 48 hours.
There are quotes in this manga and I can't always find the exact reference. If you know the exact line or quote, I'd appreciate if you could let me know!
Warning: sexual content.