Gin-san takes history lessons, and teaches you how to hide your porn mags?
Tsukuyo tries to teach Seita history, the subject he's not great at. However, her training (?) methods end up leaving him scarred, which makes desperate Seita ask Gin-san to take over as his tutor...
in order to protect those young boys' 
dreams from Sis and the others—
...من أجل حماية أحلام أولئك الشبّان من شقيقتي والآخرين
para proteger os sonhos dos 
jovens das irmãs e tudo mais—