Someone who still believe that a social ladder exists in High School and blindly follows whom they believe to be popular. They will act as if the ones they are following are superior compared to others. These sunflowers aren't considered on the same "social level" as the people they are following.
A reference to how sunflowers blindly turn towards the direction of the sun.
(Person A is the sunflower)
Person B: "Hey you want to hang out with us on Friday?"
Person A: "Oh sorry, I'm hanging out with the group."
Person A: "Oh look at Person C! She looks so awesome with her new outfit! I think I'll start buying clothes from where she buys them!"