In fiction, teens are ruled by their hormones. Be it boy or girl or something else, they want to attract attention from the opposite sex (and even both at once!). They will think about this all the time and base their actions around it.
A teenage girl automatically wants to wear provocative clothes, date sleazy guys, do poorly in school and otherwise give her father a reason to be an Overprotective Dad. If she doesn't do anything like that, she still secretly wants to. There's likely to be a secondary character avert this by being a tomboy or otherwise ostensibly uninterested in "girly" things, but even most of them secretly drool over guys, because in writer-land there's no such thing as a girl who isn't obsessed with boys (or occasionally other girls). If she's not interested in fashion at the start, she usually gets an a makeover (unnecessary or otherwise) and subsequently winds up dating the male lead.
A girl is seldom allowed to be realistically uncomfortable with her changing body, or want to maybe stay a child a little longer. In Real Life, many young teenage girls have trouble adjusting to their changing bodies and the resultant shift in attention they receive, do not look forward to having a period, and/or are simply disinterested in boys until they reach their later adolescence. In fiction, a late bloomer is almost universally used only if she's going to become interested in boys and clothes, with the unfortunate implication that there's something wrong with any girl who doesn't, or that a girl is 'incomplete' without a boy.
This is an unfortunate side effect of the Most Writers Are Male phenomenon; male writers may simply have little to no understanding about how teenage girls work. Books by female writers, especially those that are aimed at a teenage audience, can be better at averting this than adult media that contain teenage characters.
A teenage boy automatically wants to be buff, date fast girls, slack off in school and otherwise give justification for dads to be overprotective. They fall victim to obsession with the other sex, which is fairly unrealistic when the boy in question is still a preteen. Boys tend to be portrayed as spending much if not all their brain-power on getting/dating/impressing girls, when in Real Life most have hobbies and a life outside of skirt-chasing (especially younger boys, unless they're early bloomers).
A boy is seldom allowed to not be interested in sex. After all, A Man Is Not a Virgin and all boys want to become manly men as soon as possible, right? Their other interests, if they have any, are second to girl-chasing because A Man Is Always Eager. This trope comes with the unfortunate implication that there's something wrong with any boy that is not sexually active or that a boy is 'incomplete' without a girl.
If the writer is male, they may become better-thought-out characters because Most Writers Are Male. Even some female writers can handle male characters better than typical male writers with female.
(Younger) Sister Trope of All Women Are Lustful and All Men Are Perverts. As with adult characters, there's no such thing as Asexuality, and there are almost always No Bisexuals, especially among teen males. Older female teens will (very rarely) be allowed to be bi, but again that's because Most Writers Are Male. This trope comes from the same sort of mindset as Everybody Has Lots of Sex, since both tropes assume that involvement with the opposite sex is highly important to everyone, but usually not alongside it except in a particularly risqué depiction of the high school setting.
Though this is taken to severe extremes in fiction, many adults and even some teenagers (and this varies by community) will agree that this is Truth in Television. Its opposite is No Hugging, No Kissing. See also Bratty Teenage Daughter and Dumbass Teenage Son.
In Onegai! Samia-don, the early-to-mid teens Simon "Sil" Turner showed more than one shade of this. In one episode he asks the Psammead to turn him into a girl so he can chase away a boy who was interested in Anne, and in another he was furious when the Psammead got to give Anne a kiss.
In Sex Criminals, after her first orgasm somehow grants her the power to freeze time, Suzie tries to suppress her urge to masturbate until she can figure out how. This doesn't last.
Played straight in Mean Girls, of course, but the movie is also a satire.
She's All That turns the female lead partly into this, complete with Unnecessary Makeover. It still possesses a good Aesop about staying true to who you are, though, even if it's slightly undermined by the implication that you still need to look like everyone else.
The Bratz movie, especially on the fashion-obsession front.
13 Going on 30, though not so much on the dating front. Definitely fits the fashion-obsession angle, though.
Amy Dolenz's character in the Tony Danza film She's Out of Control.
Male example in The Seeker, the film adaptation of The Dark Is Rising. In the books, Will is a thoughtful eleven-year-old who's described as 'wise for his years'. In the movie, he's a fourteen-year-old Jerk Ass who immediately wants to use his newfound powers to get a girl.
The daughter in Legion before all heaven breaks loose.
Nick from Youth in Revoltreally wants to get laid. He comments when narrating about his parents and their partners that he's the only one who isn't getting any.
By means of obvious subtext, this is Older Than Print: The narrators of Boccaccio's The Decameron are three men and seven women between the ages of fourteen and twenty-seven. An inordinate number of the hundred stories involve sex in some way—including several told by the women. And most scholars agree that at a minimum, all three of the guys are actively trying to get into at least one of the girls' pants—in the case of the wit/possible Author Avatar Dioneo, he's probably trying to get into all of their pants. And some of the girls seem like they might let them. (Yes it's the Middle Ages, but it's also the height of the Plague—the kids are unsupervised and Plague does funny things to traditional morality. Also, late-medieval Florence had...unusual...sexual mores for its day.)
A couple of the actual stories have this as well.
Played straight in Dean Koontz's Phantoms, where it's specifically said that the fourteen-year-old girl is 'at that stage where most girls were obsessively concerned with boys, boys above all else' and opens the book with her arguing with her older sister about dating. She gains more personality as the story goes on.
Played extremely straight in Twilight; all Bella does is obsess over Edward and how perfect he is. Most of the other female characters aren't much better. Edward is just as bad (if not worse, given his stalker-ish tendencies), and practically every other male thinks about little besides Bella.
Are You There God? It's Me, Margaret.. In all fairness, it was written in 1970, when discussing things like periods and puberty outside of health class was still somewhat taboo. Judy Blume was somewhat notorious for tropes like this, which gave a coronary to the Moral Guardians of the day, but back then the intent was to show girls that was all OKAY.
Used heartbreakingly in the Lois Lowry YA book A Summer to Die—Molly, the elder sister, is obsessed with boys and the idea of getting married, to the severe annoyance of her younger sister Meg (who is secretly jealous of Molly's boyfriends and good looks). Molly gets sick and Meg at first resents that all her parents' attention is paid to her sister, until she realizes Molly's illness is something serious (it turns out to be leukemia) and she's going to die. Thoughts of boys and weddings help Molly keep some semblance of an idea that she's still a person, not just a terminal patient.
In the Discworld "witches" plotline, both Magrat and Verence fall under this trope. As in many of the Discworld books, it's Played for Laughs (and Verence and Magrat are both presumably out of their teens, if not by much.)
The Dresden Files (of course): Molly Carpenter is a Perky Goth version of this. When she first becomes important to the story, she's dropped out of school, gotten a bunch of tattoos and piercings, started hanging around with the wrong crowd, and dresses like, in the protagonist's words, "Frankenhooker." When asked directly if she's sexually active she admits to being a virgin but states that she has "explored" all the other bases (this is admitted to after she openly propositions Harry). She avoids going home whenever possible because any conversation she has with her mother turns into a shouting match inside of ten seconds, and develops a bit of a crush on Harry mostly because her mother hates him. She also started using Black Magic; this, naturally, does not go well. On the plus side, when she ends up as Harry's apprentice, she has to follow his rules moderating the worst of her behavior.
In The Red Tent, Dinah looks forward to having her first period (and thus becoming a full-fledged member of the Red Tent's inner circle with her mom and aunts), and undergoing the mysteriousRitual of Opening so that she is considered a woman and not a little girl. When that finally does happen, she actually looks forward to getting her period every month and the New Moon rituals in the tent done at that time.
In Bryan Miranda's The Journey to Atlantis, this is basically the main character's reason for most of what he does. Although it's not all hinged on it, it's the reason he goes to such lengths to appear to be The Hero.
For the most part averted in The Hunger Games. Katniss and Peeta do spend a whole lot of time making out but it's mostly an act because their lives depend on selling the "Star-Crossed Lovers" story. Peeta never presses his advantages when they are alone and even spends many nights sharing her bed but being perfectly chaste, and Katniss never implies that he appears to want to do more than that. The first two books only have two moments where Katniss lets her hormones take control and in the third book she's got a war to worry about (until the end where she sleeps with Peeta and then declares her love for him).
Live Action TV
That 70's Show plays this straight but lampshades it frequently. In one episode, it's shown that the only thing that doesn't turn Eric on is the thought of adults having sex — specially if said adults are his parents. (He's totally traumatised when he walks on them about to do it, and even his Jerk Ass older sister pities him for that.)
iCarly follows the "tomboy" aspect to the letter with Sam, but averts the trope as a whole, but does the occasional episode like iDate A Bad Boy or iSaved Your Life, where the trope plays out pretty spot on.
Cordelia's scenes consists of dancing, boys, and bitching at Buffy.
Buffy herself averts it, at first only because her responsibilities as Slayer prevent it, but she grows up as the series goes on.
Also, as per the current page quote, Xander.
When Buffy reads Xander's mind...
Xander: "What am I going to do? I think about sex all the time. Sex. Help. Four times five is thirty. Five times six is thirty-two... Naked girls. Naked women. Naked Buffy. Oh, stop me!"
Buffy: "God, Xander! Is that all you think about?!"
In Season 3 of Outnumbered, a BBC sitcom about a pretty realistic, rather-dysfunctional family, all Jake's storylines involve his new tendency to stare at women, making him pretty lecherous for a 14-year-old.
This is probably an attempt to add attributes and plotlines to an otherwise fairly uninteresting character, who is constantly upstaged by the other children on the show.
Blossom tended to feature this, though Six was an example of this to a larger degree than Blossom herself.
The short-lived Andy Richter sitcom Quintuplets was this way with at least three of the kids. Youngest kid Patton was a tiny ball of lust who rarely thought of anything else (his father at one point says he could "move to Hornytown and fire the mayor"), while the episode "Quint Con" reveals that both vain daughter Paige and non-conformist daughter Penny are as well, once you get them behind closed doors.
Barbie eventually grew out of this with her and her friends gaining personalities and mainly focusing on their goals. (Barbie has acting, Teresa has fashion design, Grace has sports, Summer has writing and so on.)
Likewise, the Bratz dolls advocated shopping and fashion as a cure for all of life's woes. To say nothing of their actual appearance and, uh, taste in fashion.
In Fire Emblem Awakening, while Inigo is the Handsome Lech of the Second Generation, boy!Morgan and Owain also play the trope straight once in a while. In Morgan's case, if he falls for Kjelle he ends up awkwardly admitting that he has what sounds suspiciously like erotic dreams about her, while Owain's supports with any of his dads features said dad catching him with is all but spelled to be a Raging Stiffie.
With it's premise of "umpteen high school students stuck in a relatively closed space together", it'd be hard for Dangan Ronpa to *not* have at least some of this.
Amber (who later moved to co-star in Sticky Dilly Buns) was allegedly quite a shy teenage girl, and flashbacks confirm this. However, after Zii introduced her to good sex, she got rather carried away, and ended up working as a porn star. She is in her 20s and has just turned to straight acting when she first appears in the comic's present, and while she isn't bitterly regretful, she seems somewhat embarrassed by her porn career.
With other characters in the comic, though, it gets more complicated. Details of Gary's teenage years are somewhat shadowy; he claims to have been teased mercilessly and frustratingly by teenage girls, but he also admits to having been an Internet troll, so he may have been a bit of a hormone-addled creep. DiDi, the first of her age group to get breasts (and spectacular breasts at that), doubtless attracted male interest whether she wanted it or not, but she seems to have welcomed it well enough. And Ruby of Sticky Dilly Buns fairly consciously averted the trope after a really bad experience.
Scary Go Round and its spinoffs in the Bobbinsverse, especially Bad Machinery, tend to play most with the male version; the adolescent boys in the cast are usually very susceptible to being distracted by any good-looking women or girls they see (though not always terribly successful in pursuing them). An older character says of one of them, "He's 16. His body is 73% hormones." The teenage girls in the cast are usually more rational and cool-headed, although they're not always entirely immune to the effect; along with a fair bit of snarky flirtation from the smarter ones, there's at least one teenage pregnancy reported from off-stage. Still, even when the teenage Sarah Grote becomes increasingly sexually frustrated and ends up dating a somewhat older man, she's quite sane and rational about it.