The 14 year old boy NAME_1 won a bet against his cute 13 year old little sister NAME_2, who now has to submit to his every whim. NAME_2's boobs started developing last year, were yet small. Her butt was still a small girl's butt, a tuft of hair crowned her otherwise smooth pussy. "I've got an idea for your school outfit", NAME_1 says to NAME_2. "You can wear my big hoodie like a dress but be completely naked underneath!", he says excitedly.
After school NAME_1 takes NAME_2 to discuss his grade with his math teacher, NAME_3. "NAME_3, sir, what grade did I get in the test?", NAME_1 asks. "You've got an F, why?", NAME_3 replies. NAME_1, standing behind NAME_2 starts lifting the hoodie up, slowly, revealing the top part of her thighs. NAME_2 looks off to the side, avoiding eye contact. "Oh, I see...I think your grade was...actually a D", NAME_3 says, hesitating, his eyes fixed on the freshly revealed part of NAME_2's flesh. Miraculously NAME_1's grade improves the higher the hoodie goes, culminating in an A+ when the hoodie is wrapped around NAME_2's head.
Flesh out and embelish the story. Start with the grade negotiations.