Recently I was talking with a group of teachers and the subject of inappropriate symbols came up. Specifically, a student had included a wonderful rendering of a skull in his drawing. Skulls are apparently on the prohibited list at this teacher's school. Rather than seeing an obviously well-drawn skull as a portent of social alienation and potential suicide, it could be the basis of an exciting and enlightening unit. Rather than a referral to the school psychologist, it could lead to genuine education.
Students regularly revert to clich images and symbols in their art for many reasons.
Sometimes they know there is general acceptance among their clique or peers for a certain image, such as Pokemon or cute little mousies. Other times they may simply go for simple yet acceptable no-brainers, like hearts or daisies. Occasionally, their motive is to shock or repel, as with gothic imagery or swastikas. If you get lemons, make lemonade. It is much better to pick up symbols that have obvious currency with students than to reject them as aesthetically, artistically or socially unacceptable.