Basic Trope: Catholic schoolgirls aren't as innocent as they look.
Straight: Margaret attends Our Lady of Tropes High School, and has... well, quite a bit more sexual knowledge than would be expected from someone whose sex-ed class was abstinence-only. And she's on the Pill (her mom thinks it's to regulate her periods or ease PMS, but her boyfriend knows better.)
Or the school uniforms are all the fetishized version (bare midriff, cleavage, thigh-high stockings, short plaid skirt), and the classes are taught by Naughty Nuns.
Downplayed: Although she has never actually had sex herself, Margaret secretly reads women's magazines, erotic books, Yaoi, etc. to store up information that will one day help her in bed with a boyfriend.
Teenagers are prone to experimenting with sex, whether it's approved of or not. This is especially so when (non-marital) Sex Is Evil and therefore kind of a Forbidden Fruit.
Margaret isn't a High School girl of any kind (Catholic or otherwise), but she's a stripper/porn star/prostitute/other sex worker who caters to that particular fetish.
Reconstructed: Margaret uses protection, and strikes a good balance between sexy and outwardly "respectable." (She's more of an Ethical Slut.) She also selects her partners carefully, rather than screwing Anything That Moves. If she's been abused, she seeks help from a professional.