Create a menace, the scarier the better. The best kind of danger is the kind of trouble only kids would get into or that their parents would never understand. The possibilities are endless: zombie classmates, teachers or parents; aliens impersonating classmates, teachers or parents; toxic sludge, radioactive meteors, killer clowns or mass murderers in hockey masks. What's most important is that any teen can die horribly and/or comically at any moment.
Find a theme or moral. Teen movies make at least a pretense of social redemption: the values of family or love, teen responsibility, the triumph of good over evil. The characters themselves are almost always in jeopardy because they did something stupid, irresponsible or wrong (broke curfew, smoked pot, used their parents' car without permission, explored the old mine shaft with warning signs posted, had sex).
Write your concept as a single sentence synopsis: "Masked mass murderer kills teens in while they dream of sex," "high school experiment mutates science nerd into T-Rex," "teen surfers wrangle with mutant sharks." A synopsis focuses your thinking during the screenwriting stage and becomes your pitch when its time to sell the script.
Develop your characters. They don't have to be consistent or complex, teen movies are about action and thrills, but you should give them more than a passing thought. Typical characters include the misfit, the jock and his girl, the science nerd, the party animals and the stoner. The hero or heroine is nearly always a misfit who everyone else ignores, usually to their peril.
Find a setting that makes the ordinary truly scary. Teen horror movies tend to focus on the everyday places teens hang out and relish turning safe havens into places of danger: the high school locker room, the soda shop, prom night, camp, lover's lane.
Outline your movie. Teen horror movies have three acts: act one establishes setting and character and ends with the first attack; in act two the characters realize they are under attack but can't find a way to escape or warn others; in the final act the few survivors finally overcome. In the last scene the threat reappears to set up the sequel. Don't forget to throw in some plot twists to keep your audience guessing (the boyfriend we thought was dead isn't, the monster laid eggs, the sidekick was infected when we weren't looking).
Write your script. Feel free to make changes to your outline. Write in screenplay format, which is about one minute of screen time per play. Keep the script under 90 pages; teen horror movies should never run more than 90 minutes with credits.