Seemingly-Wholesome Fifties Girl

Now in the daytime Mary Hill was a teaser
Come the night she was such a pleaser
Mary Hill was such a thrill after dark
In Cherry Hill Park!

—Billy Joe Royal, "Cherry Hill Park"

She seems to be the perfect, wholesome teenage girl of The Fifties. By day, she's a sweet, innocent ultra-virgin who dresses in knee-length skirts and tasteful sweaters, and repeatedly says things like "that's swell!" and "peachy keen!". She might wear glasses, attend Catholic school, or even be a Preacher's Kid. Her parents and teachers talk about what a "good girl" she is, but all the kids at school (except for the "squares", of course) know about her "reputation". By night, she drinks, smokes and hangs out with (gasp!) boys. She "parks" with them, while the car radio plays Nothing but Hits, and may even have experienced sexual intercourse. In other words, she's either something of a Hot Hypocrite or at the very least something rather different from what she appears to be. After all, she doesn't want to be a "square" and the male hero needs a girlfriend that appears respectable, but isn't a prude. Bonus points if her mother or grandmother was a flapper, teaching their daughters and granddaughters the way of sly rebellion and liberation.

She could be seen as a subversion of the ultra wholesomeness stereotypically associated with the Fifties, but it's more likely just an example of Truth in Television (more or less). There were large differences in premarital sex rates after the pill was introduced in 1961, and again from the sexual revolution. Despite this, by the 1970s the '50s were often just treated as the present in regard to sexual behavior, except with far more hypocrisy. Perhaps it's just the fact that the whole franchise of Fifties nostalgia films about the sex lives of teens couldn't possibly exist without her. She is a perfect combination of the nostalgic 50s charm and 1970s priorities. It is no coincidence when this trope became popular. And, of course, this type of ersatz "good girl" can still be found (or suspected) in more culturally or religiously conservative or repressive environments even in the present day.

A variant of Naughty by Night. See Hot Librarian for her more adult (and less intentionally sexy) counterpart. Contrast with the sexy-on-the-outside, innocent-on-the-inside Hooker with a Heart of Gold.

A Sub-Trope of Living a Double Life.

Examples of Seemingly-Wholesome Fifties Girl include:

Film

Played with in Pleasantville, where a girl from our day and age ends up stepping into this role... and by the time the movie's done, every girl in town is like that to some extent, except the girl who started the behavior.
Marty's mother Lorraine in Back to The Future wants to be this, to Marty's understandable alarm. Not only is she Marty's mother, she hypocritically berates her children for acting this way in the future (at least until Marty improves history).
The 1984 film Angel: "High School Honor Student by Day. Hollywood Hooker by Night." One wonders how she kept her grades up, if you know what I mean.
Janet Weiss from The Rocky Horror Picture Show. She's an innocent girl who is Brad's fiancee. Though, during her time in the castle, she has sex with both Frank N. Furter and Rocky...the latter prompting pure rage from the former. In the end, she's confused about what has occurred, but also realizes the benefits of living a sexually active life.
Subverted into a straight up Crowning Moment of Awesome and Crowning Moment of Funny when Mary Lane of Reefer Madness, the ingenue of ingenues, takes one puff of "marijewana," at which point... Some things just need to be seen.

Literature

Sybil in Invisible Man is a funny case in that she would qualify as The Ingenue if it weren't for her rape fantasies. (As the victim, mind you, not the rapist—she's utterly smothered by her life, and wants something wild.)

Live-Action TV

Subverted in the pilot of Happy Days where one girl's "reputation" turned out to have originated entirely from boys bragging about how far they'd gotten with her.

Professional Wrestling

Cherry, a manager for Deuce & Domino, a tag team of greasers. Unfortunately for Kara Slice, the girl behind the gimmick, it was her only shot at the big time. According to some sources, she was reportedly released for being "too fat" to be a ~~wrestler~~ Diva.

Music

Chuck Berry's hit "Sweet Little Sixteen" narrates a girl like this.
Then there was the aforementioned Mary Hill in Billy Joe Royal's "Cherry Hill Park". Which, because it's a fantastically catchy '60s pop tune that deals with something potentially salacious, could also be a case of "Getting Crap Past the Radar" if one isn't paying too much attention to the lyrics.

Real Life

Truth in Television. While premarital sex rates did rise dramatically in the '70s and '80s, it was fairly commonplace even in the 1950s. It reads: "Contrary to the public perception that premarital sex is much more common now than in the past, the study shows that even among women who were born in the 1940s, nearly nine in 10 had sex before marriage."
- Old quote - "Every generation thinks they invented sex."