When students at Fairfax County’s Mount Vernon High School return to classes next week, they no longer will be allowed to wear “jeggings” as pants.

What exactly are jeggings? They are the fashion cousin of leggings, the skin-tight staples found in many high school hallways. Jeggings are leggings with a faux-denim appearance, providing the tailored jean look that is in vogue among teenagers. To dress in leggings or jeggings, Mount Vernon students must wear them underneath shorts, dresses or skirts that are at most three inches above the knee, according to school regulations.

Principal Nardos King has helped to implement a more stringent dress code. Recently, the school released videos online to help students get a handle on what’s cool to wear at school, at homecoming and on prom night.

Compared with the attire of teens at other high schools in the county, Mount Vernon students’ appearance is “noticeably different,” King said. She said the dress code helps students focus on what’s important in classrooms and keeps the teenagers from being distracted by what their peers are wearing.

King said that Mount Vernon’s dress code is perhaps the only one that explicitly defines the length of dresses, shorts and skirts. King said that the “three-inch rule” helps parents and teens pick out an appropriate wardrobe for the classroom.

In a letter to parents, King wrote that the Mount Vernon administration takes proper attire seriously: “A student who violates the dress code will not be allowed to attend class unless the violation is corrected,” and “repeated violations will constitute defiance and will be subject to progressive discipline.”

King said some students complain that cheerleaders’ skirts and volleyball players’ shorts break the three-inch rule.

“Those are appropriate on the field and on the court but not in the classroom,” King said.

The rule concerning leggings as pants has sparked debate in other school districts. In Montgomery County, a Rockville High School student was forced to wear a baggy oversized T-shirt because she wore leggings and a sweater that violated the school’s dress code.

“We believe that attention to appearance and a student’s attitude toward self and school are related,” King wrote in the letter to parents. “Proper grooming and appropriateness of attire are a part of learning for all students.”