A girl that guys pay for her permission to tell people that they've performed sexual acts with her so that they get a better reputation around high school and college and their friends think highly of them. Being an Easy A also means you're not allowed to deny whether any sexual acts happened or not.
Girl 1: I heard she was his Easy A.
Girl 2: Who else's Easy A has she been?
Noun (American slang): a positive educational grade obtained by other than accepted academic means, usually via the granting of sexual favors, or other compensation, by the student to the educator who subsequently issues the grade.
Tracy was about to flunk science, until she got an Easy A after she hooked up with the science teacher.