The widely held belief that Roman Catholic school girls are sexy and/or promiscuous by their very nature. (After all, how else do you end up with all those Naughty Nuns?) Any accurate resemblance to reality is purely accidental.
As Roman Catholicism teaches that sex is to be strictly reserved as part of marriage, it is widely believed that children in Catholic families are not taught about sexuality in a way that promotes an informed understanding of themselves and their world. This feeds into an idea that since they supposedly know nothing of these things, seducing them would be a lot easier than seducing more ‘enlightened’ girls who ‘know better’.
Alternatively, even those who know differently may feel that a Catholic girl striving to be innocent is a desirable virgin, who is a greater challenge to seduce, and thus a “greater prize” if she caves under pressure. Or else there’s the notion (sometimes Truth in Television but more often wishful thinking, given the fetish-fantasy nature of the trope) that a seemingly chaste schoolgirl may only be going through the motions of chastity and religiosity for the benefit of parents and teachers, similarly to another trope somewhere around here, and is just itching for a chance to reveal her less-than-religious side the moment no one’s looking.
Regardless of the (admittedly squicky) reasoning behind it, the trope has led to the fetishization of the standard “school girl uniform”, Catholic or otherwise, frequently modified to Stripperiffic specifications. This outfit usually consists of a button-up shirt, sometimes worn with a tie (in more exploitative media, the shirt is worn without a bra, and knotted so high up that the girl in question is simultaneously baring her midriff completely and showing off a lot of cleavage); usually a blazer or jacket of some kind, frequently a couple of sizes too small; a short, pleated skirt that may or may not be in some kind of plaid; knee-high socks or stockings; and often pigtails. Expect panty shots to be a frequent occurrence (if she’s wearing any).
When this outfit shows up in films and TV shows, it’s almost always done through Dawson Casting. Likewise, when it’s done by fully-grown women, it’s almost always a case of Fanservice in action.
In Britain, where the trope is not associated with Roman Catholicism to the same extent (due to uniforms being much more common in both public and state schools), this look is known as the “St. Trinian’s Girl”, after a series of Boarding School movies by that name. Related, but not identical, to the sailor-suited Japanese schoolgirl fetish, Joshikousei. This is slowly becoming a Discredited Trope now. Slowly becoming part of the “once common but no longer generally required” subtrope of Hollywood Dress Code, since many real Catholic schools have changed their uniforms to unisex khakis and polo shirts, especially as more schools realize that what looks pure and innocent on a 9 year old takes on a different context (whether intentional or not is irrelevant) when worn by a 17 year old after the “summer growth spurt.”