I would say that Melinda herself is reminiscent of the germinating seed that she works with in her biology class, and she ‘grows’ into the tree that she produces in her art class. Her friend Ivy also symbolises growth as she reveals the normality of experiences which affect the mind with her fear of clowns.
David would be represented by a camera after his filming of the racist teacher, and also perhaps the American flag, as this represents the principles which David was standing up for.
Andy, Melinda’s attacker, is represented by blood. When Melinda spills his blood, it is a strong symbol that she has avenged his ill-treatment of her.
Mr Freeman the art teacher is referred to as a "grasshopper", but his name, like other labels Melinda gives in the text, is more indicative of who he is - a free man content in himself and driven to help others feel the same. Mr Neck is a racist bully, a stereotypical redneck hated by Melinda.