The USA has a deep culture of discomfort with the human body, and unfortunately it's so pervasive that change is unlikely. Partly this is because of the pernicious influence of neurotic religionist mythologies and partly because so many people who arrived in the USA over the centuries came from peasant origins, and peasants are typically far more afraid of physicality and sexuality than their more educated and cosmopolitan peers. The net effect is to keep the USA trapped in the mentality of the Victorian period, albeit with whizzy technological toys. It's not surprising that school dress codes should reflect the moeurs of US society as a whole, but attempting to change the superficial without addressing the underlying cause is unlikely to yield adequate results.