Editing first screenplay

Well you would probably want to spend some time around some kids that age in order to get a feel for how they talk. Which could prove to be a challenge depending on your circumstances. Anywho, remember, a good rule for dialogue is to keep it short and simple. People in the real world don't speak sentences upon sentences to each other. In fact, people usually speak with really short phrases and words.

It's difficult to give specific advice for different writers, but here is what I do whenever I write dialogue (in fact before I start writing) and I have trouble thinking of speech patterns.
First, I list the characters, then add notes about the type of personality traits I want each of them to have, what they do for a living/education standard, 'social class', and sometimes (when I really have trouble) make notes on how I imagine they view themselves.
Then it's a matter of simply trying to place myself in their shoes and thinking about how my personality would come out in my speech. OR sometimes I base it on actual people I know and think of their intonation and speech patterns when they talk in various situations, for example how do they react to shock? What is their manner when relaxed? If they're interested in someone? Sometimes you end up with dialogue you never expected to write.

Here's an example from a current project:

RINGLEADER (unnamed):
Low-level local street criminal
Brash
Tries to play up to his gang status (i.e. believes he is more of a player than he actually is)
Acts more confident than actually is, especially in front of peers
Disregard for others feelings
Self-obsessed but not to an extreme
Watches too many gangsta films, thinks he's a bigger player than he is
Doesn't like to have his confidence knocked/position challenged in front of peers (becomes embarrassed/frustrated?)

A sample of the dialogue:

EXT. PARK - GANG LOCATION.
HEAD SHOT - RINGLEADER


CUT TO:
EXT. PARK - GANG LOCATION
Other gang members turn around


A short pause


CUT TO: OVER SHOULDER - RINGLEADER


This method is useful for when you revise the screenplay. I always keep the notes handy to refer to when writing, and a quick glance refreshes the memory of what role you want the character to play (always helpful when staring at pages of writing for hours on end).

Yeah I find writing teenagers' lines pretty tough as well. It's really easy to fall into the trap of writing stereotypes (without even realising it) and I think there's often the temptation to exaggerate certain speech patterns (so lots of uses of the word 'like', excessive swearing, being grammatically incorrect etc).

I think beamazed's advice is really helpful though, you need to spend time around teens and remember to keep speech short and sweet. Maybe take a look at some dialogue from teenage characters you're impressed by? All of that helps me.