Routine
When Shouta saw his classmate Nao masturbating in the classroom, he couldn't help but ask her "Do you wanna do that... together?", and since then, they have been masturbating together in the school building early in the mornings. Seeking some excitement in their daily routine, which had become monotonous and routine, the two decide to "get down to it for real"...

(Source: PinkPineapple, translated and edited)