The PC Term is "Hymenally Challenged"
Now, if you're going to strictly define a film as a "teen movie" some guidelines will apply. For example, I do not include horror films. Horror films are rarely, at least in any way but metaphorically, about the teenage experience. Additionally, because there is a big difference between movies for teens and movies for adults about teens, I do not include nostalgic films or most indie movies, as these are not generally geared toward teenagers. So movies like Rocket Science or Brick do not count. I am on the fence about Charlie Bartlett but as it was only OK, I'm not really hung up on it. On a related note, movies about high school, but with a focus on the teacher (Dead Poet's Society, To Sir With Love) are right out.
Also, Hollywood tends to find the terms "teenager" and "high schooler" interchangeable, so any movie that takes place post-grad no longer counts as a teen film. This sounds awkwardly exclusive, but consider Say Anything. I have a really hard time describing that as a teen movie, and that is because, in the cinematic world, you go from graduation to grown-up. Simple as that.
Finally, I am unable to categorize Back to the Future unless I just include a category titled "Awesome Things."
In the most concise terms, there are two groups: the teen comedy, and the teen drama. We then have divisions within those groups:
The Slice of Life Dramedy
This will include your Fast Times At Ridgemont Highs, your The Breakfast Clubs and Some Kind of Wonderfuls, and I'd throw Pretty in Pink in there, too. They are not exactly true to life, but they come impressively close when you consider some of the rest.
The Sex Romp
The loss of one's virginity is a magical thing, and a constant source of dumb hilarity for a lot of movies. Which is OK, because frankly it's important for us to laugh at something which is often the tiniest bit mortifying. It can be awkward, all flailing body parts and nether pain and the rim of the mini-trampoline stabbing you in the hip.
Ahem. Sorry. That got away from me and became oddly specific and personal. Anyway.
The High-School-Times-100 Wackiness
This includes all my favorites. We have Sixteen Candles, Clueless, Can't Hardly Wait, Ferris Bueller's Day Off, Ten Things I Hate About You, Better Off Dead, Mean Girls and a ton more. These films get high school almost right, but just slightly more absurd (in the interest of restraint from redundancy, I explain this better in the Sixteen Candles review from '80s Week).
This division also includes a subdivision that shares some overlap with The Sex Romp: The Gender Swap. Fun fact: There has never been a genuinely good gender swap film. I don't care if the first time you saw tits was in Just One of the Guys; that doesn't make it good.
The ZOMG-I-Have-Secret-Cancer-WTF Tearjerker
In real life, do people ever hide their terminal illnesses? Because people love doing that shit in movies and TV shows. Secret Cancer is the sneakiest way for a screenwriter to show a character's lust for life and care for the happiness of those around them. Because, you see, no one can just love life that much. They have to be thisclose to death's door. This is what I affectionally call The Maude Mentality. No one is happier than dying people. See (or don't see because they're awful): A Walk To Remember, Here On Earth
The Coming of Age Life Lesson
This describes every other teen drama from Splendor in the Grass on (its lesson: hit that or DIE).
Were I better at my job, I would have put some charts together. But we'll save that for "Everyone is Drunk and Fucking Everyone: College Life In Film."*
*I was joking when I wrote this. But look for this one day when writer's block has taken hold.
After comments and sleep, I have realized the need for additional categories under the teen comedy grouping (my stance on drama remains) - I Am Turning Into a Supernatural Creature, Turning To Crime for Fun and Profit (pulling Heathers out of the HST100 category), Musicals and Dancing, and Coming Out (which, as Allie noted, are usually indie flicks, but as no other movies are geared toward gay teens, they count).