Story of a Girl ends kind of ambiguously, with a short chapter in which Deanna goes back to Terra Nova for the first day of junior year. She's written Lee a letter asking her to meet in front of the school, and Lee's there with Jason, waiting. Darren drops Deanna off and reminds her that what she said to him is true—they can be different from their dad. (Which is another way of reminding her that she needs to apologize to Lee.) When Deanna gets out of the car, Jason asks if she's ready for junior year, and they all walk into the building together.
The last line of the book is, "Lee picked up her backpack. 'Okay then," she said. 'Let's go'" (16.24). It's Lee's indirect way of forgiving Deanna, just like asking Lee to meet was Deanna's indirect way of asking forgiveness. They may not be explicit communicators, but they're making an implicit agreement to remain friends.
While it might see like a bit of a cop-out on the author's part not to show us the showdown between Lee and Deanna, it's a very true-to-life moment, just like the rest of the book. All we need to know is that they're making an effort to be friends again because they, unlike Deanna's dad, can forgive people. We see that Deanna has changed somewhat, but not that she's going to live happily ever after—that wouldn't be true to the ambiguity and hopelessness of the rest of the book, in which Deanna's just trying to figure out if anyone will ever care about her.
And on that note, we see that Jason and Lee care; we also see that Deanna is brave enough to walk through the doors of Terra Nova again. And that's the closest and we will get to a happy ending, which isn't too shabby considering how the book opens.