Juliet sits against her window seal (Did you mean seat or sill?), her legs draped (Over what?) out into the cool night air. The silk nightdress, covering her bare body, flows loosely in the wind. The evening lies in repose with the stars as the wind sings the world a lullaby. (This sentence is a little confusing.) Staring out into the opened (The opened what?), Juliet keeps her gaze ontothe dancing trees as the long, luxurious grass sways along with the wind. (Is Juliet paying attention to the grass as well?) She feels as if she is kept for rancor (Rancor is misused. Before you use a synonym from the thesaurus, it's always a good idea to look up its definition to make sure its the right word.) inside her own castle. However, there is no one watching over this thirteen-year-old child tonight as she quickly changes into the first dress she can find and slides out her window. She flies through the air, her gown flowing along with (Rather than "flowing along with", I think "floating around" works better.) her weightless body before she hits hard onto the earth. Juliet’s knees now take the color of red and black, her dress lamentably torn, but Juliet (You've already mentioned Juliet's name in this sentence. Use "she" instead.) hardly notices, her mind set on Romeo.
Juliet rushes through the waves of grass, past the creatures resting in their freedom (What do you mean by creatures resting in their freedom?), and into the starry night. Whitegentlepetals (Using several adjectives at once is called stacking. Generally speaking, stacking adjectives is not a good thing.) form a crown around her small head, a cloak around her delicate body (What is a cloak around her body?). Juliet runs without stopping; she is required (Required? Who is ordering or forcing her to meet Romeo?) to meet with her true love tonight.
When Juliet finds Romeo, he is standing with his back away from (It's easier to understand if you just say "facing".) the still lake. Juliet stays in one place, having a heart attack simply at the sight of Romeo (Make it clearer that it's not a literal heart attack.). Dozens of fireflies circle her, brightening up the darkness. “Go Juliet! Go Juliet!” they discourse (Discourse is misused.) in their high-pitched voices. Juliet breathes and makes her way over towards Romeo.
She slips her fingertips along his collar bone, signaling him to turn towards her. Without much words ("Much words" implies that Romeo said something. What did he say?), Romeo presses his lips against Juliet’s. Over a thousand butterflies evolve (Misused.) inside Juliet’s stomach, but the butterflies soon turn black as she tastes someone else’s breath upon Romeo’s tongue. Pulling away(Need a comma here.) Juliet,stares into Romeo’s wide blue eyes. His face turns sallow (He turned yellow?). She reaches for his clenched fist and carefully pulls out the proof that the perjuries (Misused. You're basically saying that the lies made under oath are true.) are true. (What is this proof?) (I suggest starting a new paragraph here.) Juliet had always thought that Romeo loved her in idolatry (It is impossible to love someone in idolatry as love and idolatry are completely different things.), but right here there is a twist in her story. The grass changes into hard pavement, the pond turns into an old apartment building, and the fireflies become obnoxious cars, driving down a nearby highway. Worn out jeans and a simple T-shirt coversJuliet’s chilled body. Her rash (How can hair be impulsive or careless?) brown hair flies around wildly in the hard winds as a tear stains her red cheek. Juliet swallows the pain as it eats her alive. (Cliche imagery. Try describing to us exactly what Juliet is feeling, and how.) Backing out of the abandoned parking lot, she runs back home, regrettingwhenthat she has to tell her family they were right about Romeo all along.