When attending an event that has a casual dress code, a teen girl should choose an outfit that is comfortable and suitable for the occasion. This could be a simple sundress or jeans with a nice top. If the weather is cooler, she might want to choose a cardigan or light jacket to keep warm. She can also add accessories such as earrings, necklaces, or a scarf to complete the look. No matter what she chooses, it should be something that she feels comfortable and confident in.