PLAISTOW, N.H. — As the sun slips behind the clouds, a cool breeze makes the leaves dance and chills the air. But Elizabeth Skerry, engrossed in a subject that has preoccupied her for the past month, seems not to notice the fading light and plummeting temperatures.

Dressed in a cap-sleeve blouse and black knee-skimming skirt, she is talking about the new student dress code at her high school. The policy, drafted by faculty over the summer, expressly prohibits sleeveless shirts.