writing a screenplay any tips on how to write a compelling story?

How to get better? Practice :D

But you don't have time for that for a school assignment, so...

Structure is key in a screenplay. Absolutely vital. I can't stress it enough. There's no time for backstory or getting to know the characters. You don't say whether you're writing a full-length script (90-120 pages) or a short. In a full-length screenplay, you must grab your audience within the first ten pages and have your inciting incident by p15 latest. Think about Juno. We don't hang around getting to know the characters and seeing the relationship between her and whatsisname develop - we get straight in when she's buying a pregnancy test, and we find out why later.

Because you have to get a lot of information across fast, every word counts. Make sure your dialogue gets as much across as possible. I don't mean the characters should say a lot, I mean the way they say it should reveal a lot about them. In the same way, keep your description and action lines/paragraphs really short, only telling what's absolutely necessary and choosing strong words for maximum impact. Keep action lines verb-orientated. Adjectives and adverbs should be used incredibly sparingly.

And make sure your story is strong in essence. Story is this:

first quarter: a character has a problem and must do something about it

second and third quarters: he/she tries to fix it but things get in their way, the situation gets worse and the stakes get higher

final quarter: the character risks everything in a final effort, and finally succeeds or fails.

There should be conflict in every scene. If there isn't conflict, put some in, and your story will be stronger and your characters more vibrant right away.

If you're still stuck, look at your beginning as it is right now... Now remove the first few pages and find a later point to start. It doesn't matter if not everything adds up right away - you can fill in the details subtly later. Better?

Hope this helps.

You could start with a character who has a belief but is proven wrong in the end by the actions of another character, or perhaps concentrating on the relationship between two characters and through out the story gradually develop and reveal it and let it drive the story by considering how this relationship would control the characters actions. That sentence is a run on, now isn't it? xD

National Script Writing Month! Google it!