...which begs the questions in my mind...just when and why did Catholic schools decide to adopt tartan as girls' uniform material and when did society at large start identifying tartan as "girls' school unifrom material" and not as Scottish kilt material? Doesn't seem like you see many people walking up to a kid in a plaid jumper and saying, "Oh...you look like a Scotsman!"
The way I understand it is that, during England's Victorian era, boys wore kilt outfits at the best schools. When girls began attending such schools, they adopted the same uniforms the boys wore. Catholic girls' schools kept up the tradition after it was largely dropped elsewhere.

In other words, Catholic school girls are dressed like Victorian era school boys.