A Catholic school uniform in North America, and especially the United States, stereotypically consists of a pleated and plaid skirt or jumper (a sleeveless dress), Mary Jane or saddle shoes, a button-down shirt, and a sweater for girls, while boys' uniforms consist of a button-down shirt, a necktie, and dark pants. Actual school uniforms vary widely by location and individual school.

In contrast to most schools in the United States, almost all Catholic schools in that country have some form of dress code, and most of them (especially those with students in the lower grade levels) have a mandatory uniform policy.

The concept is not easily transposed to other countries. In most Commonwealth countries, school uniforms (often similar to the "Catholic school uniform" stereotype in the United States) are common in all types of schools, whether sectarian or not and regardless of religious denomination.

Purpose of school uniforms

The stated purpose of uniforms, often set forth in school uniform policies, include reducing clothing expenditures for parents as well as avoiding distinctions among children based on whose parents can afford to buy them fashionable clothing to wear to school. The school attire is also said to reduce distractions and help with student identification, ensuring that a stranger will stand out among the uniformed students.

In North America

Until the early to mid-1970s, the uniform for girls almost always consisted of a skirt or jumper; but it is now common, in the United States, for female pupils to wear uniform shorts or slacks, depending on the weather. This is mainly the result of changing societal norms that, beginning in the late 1960s to early 1970s, resulted in a trend for women and girls in most levels of society to wear trousers and shorts, instead of skirts and dresses, for everyday life. Today, most schools require girls to wear skirts, while others allow girls the choice of skirts or culottes and pants including ankle pants that stop at or just a little above the ankle.

A kilted skirt is sometimes worn, especially in schools with predominantly Celtic student populations. In some parts of Canada, the skirt has been modified to include an attached pair of shorts beneath, for modesty called a skort. Bike shorts or Leggings are often worn under girls' skirts or jumpers for modesty and to prevent individuals from looking under the girl's skirt or dress. The leggings can be ankle length or capri length with crew or knee-high socks worn over the ankle leggings. Culottes or ankle pants that stop at or just above the ankle are also sometimes substituted for a skirt, especially at non-parochial private schools; however, for special events, the skirt is usually still worn.

Students in preschool and, less frequently, kindergarten, are held to be more likely to engage in messy activities. As such, they may be asked to wear their school's physical education uniform as their regular uniform. Younger girls are often seen wearing plaid jumpers over a blouse under their skirts. Many schools require a jumper up to the fourth, fifth, or sixth grade, presumably because the lifestyles and habits of younger girls may make blouses more likely to become untucked from skirt or culotte waistbands. After that, the jumper is often discarded in favor of a plaid skirt with bike shorts, capri or ankle length leggings, with crew or knee socks over the ankle leggings, tights or culottes and blouse or polo depending on the weather and season. Often the skirt and blouse or polo must be worn with a sweater or sweater vest, or the polo is worn by itself or with a layering long-sleeved tee underneath or with the polo worn underneath a sweater or sweatshirt with the polo collar turned over the sweater or sweatshirt collar so it's visible.

Boys typically wear a collared shirt, tie, and slacks of required colors, although polo shirts have to some extent replaced dress shirt. In the winter colder months the polo is seen worn with a layered long-sleeved tee underneath or a sweatshirt or sweater over the polo with the collar of the polo folded over the collar of the sweatshirt or sweater so it's visible.

Both sexes usually wear a sweater or blazer (or both) or polo with a layered long-sleeved tee underneath or a sweatshirt or sweater over the polo with the collar of the polo folded over the collar of the sweatshirt sweater so the polo collar is visible when required by regulations or weather.

Some schools have unisex uniforms—most often a distinctive shirt, and sometimes pants of a given color. They may also require that a specially-designated, more formal uniform be worn on liturgy days or other special occasions.

Uniforms may vary based on time of year. At many schools, students are excused from having to wear the fairly formal (and warm) uniforms described above during the hotter months of the school year in favor of lighter uniform clothing such as skorts, skirts, or shorts worn with knee socks, crew socks, quarter socks, or triple roll socks, or capri leggings under the skirt and capri pants are also worn. However, at some schools appearance and formality prevail over comfort and students must suffer through the warm weather in their full uniform, which can include long-sleeved dress shirts, ties, wool sweaters, blazers, wool skirts or jumpers, leggings and tights.

In many schools ballet flats for girls and Sperry Top-Siders for both girls and boys have become popular and acceptable footwear with the uniforms.

Scrunchies, headbands or headwrap style headbands are usually worn in school colors and patterns.

In Popular Culture

A schoolgirl uniform fetish is a sexual fetish in which someone derives sexual pleasure from viewing others dressed in the typical uniform of a schoolgirl (with either a school skirt or culottes), or from themselves dressing in that manner.

• The hit 1998 music video for Britney Spears' first single, "...Baby One More Time" featured Spears and a number of backup dancers wearing a modified version of the Catholic school uniform.

• The Russian pop duo t.A.T.u. also wore clothing resembling Catholic school uniforms in a video for their song "Ya Soshla S Uma" (also known by the title of its English-language version, "All the Things She Said").