Kuroki Level is a reference to Tomoko's bad class introduction at the start of her second year at high school depicted in Chapter 39.
During the class introductions at the start of the second year, her classmate Hina Nemoto tells Tomoko she told the class about her and praised her for her first-year introduction. This increases Tomoko's anxiety, and she can only blurt out that she is an "F-cup." Later, Tomoko hears a boy describe his failure at something as "Kuroki Level."However, since Chapter 39 this term has not been used.