Shin Sei Yariman Gakuen Enkou Nikki

Based on a doujin game by Orcsoft. Saiki Yasuo is a 45 years old office worker. After 20 years of marriage, his wife asked for a divorce and left him. Now, depressed, he spends the nights drinking alone, until one night a girl from the St. Marian Academy approaches him looking for a client. (Source: English DLSite)