There's no definitive, readily available statistic on the exact percentage of US schools with mandatory dress codes. Data collection on this specific aspect of school policy is not consistently tracked at the national level. While many schools *do* have dress codes, the degree of strictness varies widely, and what constitutes a "mandatory" dress code is also subjective (some might have guidelines, others strict rules with punishments).
To find a precise percentage would require a massive, nationwide survey of all schools, which hasn't been conducted.