|Wow, that is some powerful, powerful stuff. You do a great job with the dialogue and you have created some real, believeable characters in Tom and Sarah. And the ending- wow, you just blow the reader away with it. Great climax, and way to pull the reader in with the intense scene and crisp writing.
But you're offering me a truckload of GPs for this review, so I'm not going to just pat you on the back and say that it's great. Don't get me wrong, it's a good story, but there are a few things I want to bring to your attention. The first is the sentence "It was the day before Christmas break; a Christmas they would never see." I really don't think that what follows the semi-colon is necessary. It foreshadows too much, and takes a lot of intensity out of the climax, because the reader already knows what is going to happen. You get rid of those six words, and the story takes on new life, because the reader does not know what will happen next.
Second, the conversation that the two of them have about Tom phoning Sarah at his dad's seemed out-of-place; they're going to be seeing each other in the evening for the movie, but they're talking as if they will depart from each other at any second.
Third, the whole memory-scene feels awkward. It's clear that you're trying to have a flashback, like in TV shows, but to just put it in the middle of things is confusing and does not flow well at all. Maybe adding in some asterics as scene-dividers will help.
Finally, while the dialogue is good, you can easily get rid of many of the "he said"s and "she said"s. They're the only two characters, so we don't need to be reminded of who is saying what each time. On that topic, turn "the boy said panic-stricken." to "said the panic-striken boy."
That's about it. You're a great writer, and I think some touch-ups can really make this story publication-worthy.