##Part I: Background and Context Great question! There are a few different threads (ha! history pun!) to pull on in your question. First, without getting too much into current events, a cursory review of Catholic school uniform vendors shows a plaid skirt is one of a handful of "default" Catholic school uniform skirts/uniforms. Plain blue jumpers or skirts or khakis appear to be just as popular and common. That said, plain blue/black or white tops and khaki bottoms are also popular in charter schools and non-Catholic private religious schools.

Which is to say, seeing a child wearing a plaid/tartan skirt or jumper (high-waisted, sleeveless dress that usually ends at or just above the knees) and thinking “Catholic school student” is a function of a few different influences. There is the feedback loop reinforced by pop culture (the movie Girls Just Want to Have Fun, the music video Hit Me, Baby, One More Time, and the SNL character Mary Katherine Gallagher come to mind) but that loop was started by the purposeful decisions by adults running Catholic schools to create a visual identity for their students. In her book, Common Threads: A Cultural History of Clothing in American Catholicism, historian Sally Dwyer-McNulty describes the most familiar ensemble as a "Catholic look." It wasn't always that way, though.

Jumping back to the early 1800s and the rise of public schools in America (then referred to as "common schools"), there was a distinct Protestant shape and form to the routines and norms in the schools. One key change was around the role of the teacher. Through a campaign across New England states led by advocates, the position went through a feminization process - rather than teaching being a short-term gig for young men in-between or on their way to other things, early advocates pushed to reframe teaching as something young, unmarried women could do as service to their country, faith, and future as a mother and wife. Their brothers could preach from the pulpit - these young women could have a classroom. In addition to their own cultural touchstones, most carried routines from Protestanism with them. For many, this meant creating a culture where children were expected to follow an adult's direction, misbehavior could be punished, and religious texts were a routine part of the day. (A bit more here about the early history of American schools as it relates to the lack of uniforms in American public schools.)

Up to and through the end of the 19th century, tax-payer funded grammar schools (1st to 8th grade or so, before the creation of Kindergarten) sprung up across the country, offering a limited liberal arts curriculum - reading, math, some history, some science, and music and art. Embedded within all of these activities were religious songs and texts. Sometimes it was explicit as part of the morning routine or embedded in texts the children would read. (It wouldn't be until the modern era that American public schools were fully secularized - however, the legality of explicit religious practices in some Southern schools is still working its way through the courts.) Because you asked about the plaid skirt or jumper, an article of clothing worn by girls attending Catholic schools, I'm going to focus on girls' education from this point forward.

While people of all faiths, including Catholics, have long been a part of American history, most people in positions of power were Protestant (mostly men, mostly white, mostly non-disabled.) As schools began to merge into school districts, especially in urban areas, Catholics raised concerns about the goals of the "common" education and the nature of the education their daughters were experiencing in school. During some of the debates in New York City, these concerns rose to the level of protests and in some cases, violence. As is the norm, there were multiple stressors leading to the conflict but when it was over, NYC had basically two parallel school systems - the public, tax-payer-funded Protestant-ish schools and the private, diocesan and tuition-funded, Catholic schools. Similar events - often spurred by anti-Catholic sentiment - led to similar systems in Boston and Philadelphia. Whereas public schools had soft gender segregation (separate bathrooms, entrances, recesses), Catholic dioceses often made the choice to adopt hard gender segregation and created separate schools for boys and girls.

Catholic, private, or tuition-funded schools in northeast states weren't new; Catholics played a significant role in the Indian Boarding Schools and charity schools - also known as academies - associated with orphanages or homes or children. Girls who attended these schools typically dressed the same as their clothes were provided by the school and in many cases, their outfits, often made by students of the school, were similar in color and appearance to the habits or clothing worn by the nuns who ran the school. In effect, their outfits identified them as a ward of the school, as parent-less. Catholic parents who paid tuition were typically sending their daughters to a private boarding academy that focused on a finishing school-type curriculum. Most of these schools were more interested in teaching girls how to be successful in polite society and would often enroll Protestant and Catholic girls and religious education would be secondary to other instruction. While some of the schools encouraged students to dress in particular colors or styles, there wasn't a uniform to speak of for daily wear as mentioned earlier, a uniform typically signaled that the child wearing it was destitute, a ward of the Church, or otherwise couldn't be trusted to dress themselves. That said, some of the tuition-based select schools were located next door or in even the same building as a "free" Catholic school that offered a religious education to children and in those instances, the girls attending the tuition school often adopted a particular look or style which would serve to distinguish themselves from the students who attended the free school. These looks and styles, though, were expected to be simple, plain, and modest. Girls at the select schools were expected to conceal their wealth and were, on occasion, expected to wear a secular version of the clothing worn by the religious who ran the school. To borrow a phrase from Dwyer-McNulty, those who set the rules around what girls were expected to wear to a school run by Catholics walked a very fine line around encouraging humility, honoring parents' wishes, and communicating class.

As cities and communities experienced a spike in nativist antagonism, tuition-based Catholic schools experienced drops in enrollment or made changes to their policies around their students' dress in order to limit a child's public identification as Catholic. Some Catholic parents elected to send their children to a public school, regardless of the religious content, as a way to protect them from harm but that quickly came to an end in 1884 when Catholic bishops and church leaders began mandating parents send their child to a Catholic school. From Dwyer-McNulty's book (p. 93):

At the same Plenary Council that ruled on Roman collars for priests, the bishops determined that each pastor must provide a Catholic school (or Catholic education, in the case of compromise plans) in his parish. Once the school was available, the church obligated parents to send their children to it.

In 1887, the Bishop in Detroit told priests to deny the sacrament to parents who sent their child to the local public school unless the parent had a good reason not to. (Few reasons - distance being one of the few good ones - were seen as acceptable.) Similar messaging took place across the country and it worked - enrollment in Catholic schools increased sharply. There's a whole bunch of other fascinating histories about the rise of parochial schools, including the impact on religious women and their role in the church but to cut to the chase, the shift towards the "Catholic look" began in the early 1900s and would be firmly established by the 1960s.