The Idol Girl in My Class Is Acting Suspiciously is a Manga/Manhwa/Manhua in english language, ROMANCE series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Saegusa, who retired from idolry when she went to high school, is good-looking and intelligent. She is always cheerful and friendly, and is the most popular person at school. Takuya, however, knows a certain side of the highbrow Miss Saegusa. Today, too, a suspicious-looking woman in a dubious disguise appears at the convenience store where he works part-time. She is, in all likelihood, that Saegusa-san...? The long-awaited comic adaptation of the hugely popular romantic comedy with an overly cute, suspicious heroine!