「学園の廊下で、濃厚なキスシーン・・・聞いたぞ聞いたぞ」「濃厚じゃなーい！話に尾ひれ付いてるって・・・」

「、・・・」「！・・・」

'A hot kiss scene in the school hallway... I heard, I heard!' 'It wasn't hot! The story's being exaggerated...'

Literal

''School-grounds [possessive-の] hallway [locative-で], steamy kiss-scene... heard [emphatic-ぞ] heard [emphatic-ぞ]'' ''Steamy not-is! Story [locative-に] tail-fin attaches [quotative-って]...''

A gossipy teen exchange. 濃厚 (のうこう) literally 'thick-dense', na-adjective for 'rich, thick, steamy, intense' — used for food, flavor, and intimate moments. 尾ひれ付く ('fins and tails attach') is an idiom meaning 'a story gets exaggerated, embellished with extras' — as if a plain fish grows fins and a tail. The repeated 聞いたぞ adds to the teasing/gossipy tone.