Tokidoki, Sekai wa Utsukushii

High school senior Yokozeki Haruhito has always been watching the teacher-in-training Arisawa Yuya. However, this doesn't go unnoticed by Yuya's cousin, Yuma. Yuma volunteers to be Yuya's substitute. Things start to escalate when one day Yuya walks in on them kissing...

(Source: Gentosha, translated)