An OVA included with the special edition version of the fourth manga volume.

Sayama is a pessimistic guy who always wears a face mask to school. When the most popular boy in class, Saikawa, rips off the mask out of curiosity, Sayama flips out. Despite his harsh words, he's somehow captured Saikawa's interest, who offers to tutor him with a bizarre condition: to let him do as he pleases with what's under Sayama's mask. Sayama hates it when people touch his bare face, lips, and mouth, but he can't resist Saikawa's attention...

(Source: Animate International)