The 14 year old boy NAME_1 is failing math and gets an idea to improve his grades. After school NAME_1 takes NAME_2, his cute 13 year old little sister, to discuss his grade with his pervy math teacher, NAME_3, in his office. 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. A perfect NAME_4, a teacher can't resist. NAME_2 is wearing just NAME_1's big hoodie like a dress, with nothing underneath.
"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, stepping behind NAME_2, slowly starts lifting the hoodie up, 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 slowly, his eyes fixed on NAME_2's thighs, waiting to see more of the teen. NAME_3 keeps improving NAME_1's grade the higher the hoodie goes. A 'B' when the hoodie is waist high, revealing her cute smooth pussy. Culminating in an 'A+' when the hoodie is wrapped around NAME_2's head, her naked teen body fully on display for excited NAME_3.
Flesh out and embelish the story. Start with the grade negotiations.