Close Range Love volume 4
The more the story progresses, the crazier the characters act in this, which works to make it better than I would have given it credit when I started reading.
I guess that when you have a plot in which a cold, genius female student gets into a relationship with the young, funny and charming teacher, keeping the humour in is the way to go to avoid the whole affair from turning into an over dramatic sap story at every turn.
So, when the ex GF turns up, it's only natural to have her be the ex-teacher of the young male teacher, who got into a relationship with him when he was a 17 year old student of hers, and even more natural that she gets in competition with our genius protagonist to try and get her ex back, acting just as childishly as one would expect an adult to act in a story where no adult acts like an adult would.
Basically, this is becoming way more interesting than I anticipated previously.