Jess's action in throwing away the art supplies Leslie had given him is a complicated one, born of many impulses. Foremost is probably anger, anger with Leslie for leaving him behind to struggle through the rest of his life. This is evident in his attack on May Belle and the furious energy which drives him. Perhaps he feels that by getting rid of the last tangible element of his friendship with Leslie, he will be able to cut her out of his heart as well. Then, too, there is the symbolism of his throwing the paints and paper into the creek where Leslie died. It is almost seems as if he is giving them back to her, canceling his debt of friendship to her, proclaiming that it is as dead as Leslie herself. Last, it seems to be a declaration that all the talent and uniqueness in him were dependent on her. Without her, he is just a stupid little fifth-grader again, a crazy little kid who likes to draw instead of the king of Terabithia and a soon-to-be-world-famous artist. Jess feels that Leslie has drawn him up to higher levels personally, and now that he is gone, he seems to feel that he cannot maintain a claim on those new parts of himself. The paint set is a symbol of this, and no doubt that it is part of why he throws it away.