You all need to learn more about 1st amendment rights in schools. I know this for a fact. It is unconstitutional for our school to ban yoga pants, just like it is for my school to ban bandanas.
Students in public schools have freedom of expression, providing their expression doesn't cause a more than a "minor disturbance." Therefore, a student can wear what they want, say what they want, etc; even if it causes a minor disturbance.
In order to ban yoga pants, the school would need to prove that yoga pants actually cause at least a moderate disturbance.
However, enforcement of this is very hard, since basically no student or other person would go through the trouble and legal fees to sue the school.