The old fingertip test to measure length of shorts and skirts is out in a growing number of school districts, while short shorts, tube tops, pajamas and attire showing cleavage are in.

Clothing once considered taboo is now permitted as more districts across the U.S. relax student dress code policies, deemed disproportionately targeted at females, and move to gender-neutral or equitable dress codes. The districts also want to end body shaming, causing humiliation by criticizing a person’s body shape or size.

“A...