I think the key here is that this is effectively a government publication - I don't think the government has much business dictating fashion trends. And being government products, schools should be exemplars of the limits on government power that we enjoy.
Many school policies purport to prohibit all kinds of speech, including t shirts with guns, NRA type advocacy, etc in addition to the immodesty malarkey. The fact that these policies are common doesn't make them any less statist - if everyone's money is paying for the school, it ought to have policies on speech and expression that are content neutral.
This perfectly illustrates my earlier point. The fact that a mode of dress can be called speech or expression is seen as an excuse to ignore rules that are put in place specifically to prohibit that so-called expression. It is precisely because the human body is not content-neutral, that it is to be covered. Yet the reason for the rule is now paraded before us as the reason why the rule must be abridged.
If such thinking as we see above is carried through consistently, one wonders how public schools are to keep students from wearing transparent garments, or any clothing at all. Once we get rid of that immodesty malarkey, that is.
"Immodesty malarkey." There's a good turn of phrase to consider. The reasonable is "malarkey." The barbaric is somehow sensible in this absurd and idiotic new world.
The first amendment has been curtailed for students, which would make this a dicey proposition in court for either party.
The first amendment has been curtailed, as you put it, for military members and other government employees as well. Are you in favor of abolishing those rules?