Toshishita Danshi wa Mate ga Dekinai

From Shinmakoku Scanlations :

Sagami's little sister suddenly couldn't make it to her omiai meeting...! Headed to the meeting place to apologize on her behalf, Sagami met the last person he expected to see. It was Uehara, his senpai from college, the guy he used to sleep with. Each word he spoke brought back memories from that time... A story of an ended love and sensitive romance suddenly revived.

This book also contains 3 cute stories of a Student Council's members.