Houkago no Love Call

A highschool teacher, Ueda Wataru, was rejected by a person he loved back when he was a highschool student and has been timid ever since. However, he's now fallen in love with his student's father, Ippongi Kazushi, a yakuza. Although Ueda knows that he shouldn't be, he's fascinated with the good looking Ippongi's sweet words.
(Source: Dangerous Pleasure)