Review the school dress code with your child and be clear that you will uphold it. If he doesn’t like it, encourage him to take it up with the principal—not you.
This should be a fight ONLY about appropriateness, not style choice. This is important! Part of growing up includes experimenting with different styles of dress, which may include fashion you don’t like—maybe even because you don’t like it. Rejection of otherwise school-appropriate choices may make your child feel that you reject her very self.
Choose outfits the night before if you find yourself having daily arguments with your child. With younger kids you might even be able to pick out an entire week of outfits.
Keep school and weekend clothes separated from each other if your child is especially resistant to the rules. It can be helpful to create a divider in the closet and organize drawers according to use.
I know that, for parents, the clothing battle can be exhausting—but it represents something much, much bigger. Teaching your child to be resilient to media and peer pressures, and to respect appropriate authority, are critical lessons that will last a lifetime. So, go to battle—you are not alone!