From strictly a fashion standpoint, the plaid skirts are cuter than the replacement outfit proposed, IMO. Also, the parents are paying for the brand and want the uniform to be recognizable, I suppose.
From a practical standpoint, I was always wondering if the girls were cold during the winter months in those skirts and tights vs. pants. I personally tend to wear slacks more during the colder months and switch to dresses and skirts/blouses during the warmer months. In the old country we had to wear very itchy and ugly brown wool dresses with white aprons on special occasions and black aprons on regular days. Boys wore white or blue shirts and blue pants and jackets. Gawd, I hated that dress, made us look like housemaids. However, during the harsh winters, I had to wear an additional pair of knit tights, and so am wondering about girls wearing skirts here.