I appreciate that the author wrote this story when she was 16-18 years old, but, honestly, it reads as though written by a ten-year-old. Choppy, repetitive language, frequent overuse of certain words (especially "beautiful"), frequent mis-use of words (e.g., "dogged" instead of "dodged"), run-on scenes, extremely simplistic action sequences (and the simplistic triviality with which most of the protagonist's fights resolve), etc.
Interestingly, Ms. King's self-review posted here on this book on Goodreads, short as it is in comparison, is much better written than any part of this book. Perhaps her writing has matured in the intervening two years. I hope so. She certainly seems to have the drive to get there, and that's often the most vital ingredient.
Along the way, if she hasn't already, I'd suggest a few things:
Vocabulary. Be careful of overusing the same words sooo often, and ensure your understanding of the words you do use. Having a dictionary nearby while proofreading your own work helps, but the best way of broadening and strengthening your own vocabulary is almost always to read. A lot. And take the time to be sure you understand every word you read, even if you have to keep a dictionary nearby while you read.
Get to *know* the phrase "Show, don't tell." It's not just a Rush song (although that's worth a listen, too), but a common writers' mantra that's utterly vital to delivering setting, scene, and story smoothly and clearly. Look up the phrase anywhere (even Wikipedia) and absorb it deeply. For example, this concept could be especially important in conveying the utter alien newness of Meridian: don't keep telling me over and over how alien and beautiful it is, describe to me what it is that is so alien and/or so beautiful that I'll be able to see that it is without you haven't to tell me that it is.)
There's obviously all kinds of more, but those two pillars stick out the most strongly to me, and I suspect that working long on those will cause many other gaps will fill themselves in along the way.
In short, IMO, the high-level story concept and outline of "All Light Will Fall" wasn't bad at all, but the experience and skill to tell it well just wasn't there at all. (Yet?)