Had same reaction and thought I skipped/forgot way too much lmao...
When I saw the black hair with weird ahoge I knew I recognized this crap though its from "We can't study" a shounen jump weekly comic you'd see in a collected weekly book with stuff like naruto bleach etc.
Someone please fix this mistake from the chapter listings lol
http://www.mangago.me/read-manga/bokutachi_wa_benkyou_ga_dekinai/
Rejoin peasants for I'm god here to answer ur prayer
sorry that went over my head , But plz do read it is a good manga
Chapter 15 is the wrong chapter. It's from a different manga.