Dress the part
As a former middle school teacher, I saw a difference when the kids wore uniforms and when we had out of uniform days in regards to their behavior. I think it's the same principle for adults going to work; you should dress respectfully for the job. It also helps that some kids can't afford brand name clothes.
The only downside I see is that kids don't really learn how to dress themselves if they're always told what to wear. The private school in our neighborhood was K-8 so most of them went to public high school with us. You could always tell who they were because they dressed differently. It was like we all had 8 years of experience with schoolyard fashion trends that they didn't. I remember one girl wearing a winnie the poo sweatshirt, not thinking that high schoolers probably wouldn't be into winnie the pool. Or a guy that had the most perfect outfits... and wore them in a perfect rotation just like he did with his old uniform
Learning to dress yourself appropriately, learning your own style, and learning how to read fashion trends is something that non-uniform kids start learning in Kindergarten. It's a trial and error thing that uniform kids just don't get.