Info 66: Dresscode at schools, no bibs allowed?

I found by coincidence an image of a girl in bibs (not with Google), which leads to a dresscode (for girls) page of a US Christian school.

Jeans, also colored jeans are allowed on elementary but not secondary class. No acid wash denim. Pants should not be too baggy or too tight. Shorts, leggings and lycra pants are not allowed, so bib overalls (neither elementary nor secondary). Proper undegarments must be worn. Also restrictions on hair (color and styles), shoes, hats, jewelery etc. Tattoes are allowed, but must be covered.

At middle or High School no jeans etc.

I wonder why they don't introduce school uniforms. A pic at the main website shows happy kids playing in jeans.

However, I search on the school's website and can now only find text and not the restriction of bibs anymore. But

• Dress should be modest. (I Timothy 2:9)
• Dress should properly differentiate between the sexes