From Renta!:

Chiaki left his phone at school, so he goes back that evening to retrieve it. While he's there, he hears soft moaning coming from a nearby room and decides to take a look, only to find Mr. Chiba, the math teacher, taking it from another man... Despite being geeky, all the girls love Mr. Chiba, who in turn constantly nags Chiaki about getting more tutoring. Although hesitant, Chiaki takes this chance to blackmail Mr. Chiba and snaps a picture of the two men in action. The next day, Chiaki shows Mr. Chiba the photo during his tutoring sesssion after school. Instead of freaking out, Mr. Chiba simply smiles and kneels down, then takes Chiaki in his mouth to "bribe" him! This is the start of extra-curricular lessons between a cocky fool and a two-faced teacher!