"The administration retains full discretion in determining the appropriateness of school dress and its effect on the instructional environment."

For example, say a girl is wearing leggings and a t-shirt, and a teacher tries to make her change.. Wouldn't they need evidence to prove that certain clothing has a negative effect on the instructional environment if it's not already specifically written in the handbook? "Full discretion" does not mean ultimate authority without evidence.

Am I correct? I'm trying to get to the bottom of some things.. There's a fuzzy line between dress codes and the actual laws.
It's very vague, though.
Laws and people's rights. Excuse me for forgetting that part.