Himitsu no Otsukiai wo Shiteimasu.

Takase, a second-year high school student, is dating a popular person in the same class, Togawa. However, the relationship between the two remains a secret to everyone. They're cool in front of their friends, but Takase is always thrilled by Togawa, who becomes lover-like when they're alone. However, one day, Kurono, a senior in the third grade, finds out that they are dating! Moreover, Kurono is interested in Takase...!? A wholesome story of a doting boyfriend and a slightly ditzy girlfriend.

(Source: Frontier Works, translated)