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...
That said, putting a padlock on 
the door would be too suspicious.
سيكون وضع قفل على الباب مثيراً للشبهة بلا ريب
Cerrar tu cuarto con candado 
sería muy sospechoso.
Dito isso, colocar um cadeado na
 porta do quarto seria suspeito.