Review from my 12 yo stepdaughter: Her review: "I really enjoyed reading The Boy in the Red Dress. I liked it because it took time and character development to solve the mystery. I would recommend this book to all high schoolers and middle schoolers."
Seriously - She loved this book. She kept talking about her theories about the mystery and was genuinely surprised by the resolution. She usually has a YA mystery solved pretty quickly, so she was impressed that the Boy in the Red Dress kept her guessing.