It sounds great. Here are a few things to think about.
You're always going to understand your characters more than the audience will, even if the movie turns out 100% perfect. So when you have relationships like Jessie's and Sam's, where they really are friends, but part of the reason they're real friends is because Jessie needs the car -- that's totally realistic, but you just have to be careful when you're writing to make sure the audience has what they need to understand the relationship (or figure it out as the movie proceeds). You have a lot of grey areas in this story, 'kinda-maybe' areas, that are great but are also places where you'll need to tread carefully. And every now and then, you'll have to make a few things just a little more clearly defined, and it sucks, but it'll end up being for the better.
Don't eliminate sex, because that's a huge part of being a teenager. It doesn't just encapsulate sex, but fear of sex, dismissal of sex, misunderstanding of sex, etc. Masturbation doesn't just have to be a joke like it is in American Pie; it can be scary or romantic or disturbing or sad. I think this would be something that you could deal with once Alice and Sam are in the car together, going wherever it is that they go.
About Jessie being beat up...one thing to think about...when I was in high school, this guy's girl cheated on him, so he and his friends got together and beat HER up. It was totally jacked up. But as far as a movie goes, it's an interesting twist on an old story.
There were other things other people replied to that I was going to comment on, but I've forgotten them...so I'm going to go read back on them now.