In Japanese, 服 (fuku) simply means clothing. In the case of an individual set of clothes, we may safely read this as outfit.
Sailor Fuku = sailor outfit.
The “sailor outfit” came to Japan in the early 1920’s. Although a different school claims to have invented the sailor fuku first, the sounder claim rests with Fukuoka Jo Gakuin (a university for women, hence the “jo” part, 女 (woman)), where the principal, Elisabeth Lee, modeled the uniform after the sailor uniforms of the Royal Navy (the name of the navy of the United Kingdom).
In light of the fact that the gakuran, the stereotypical “male Japanese student uniform” used in Japan, is actually based on Prussian military uniforms of the time, uniforms reflecting a) the concept of military uniformity, b) the fact of being Western clothing, makes clear the cultural context.
In fact, the Japanese for gakuran, 学ラン, is “gaku” for “study” plus “ran,” representing a pre-modern Japanese term for “the West.”
So, both the gakuran and the sailor fuku are intended to evoke Western modernity mixed with military uniformity.
Things have come a long way since then.
Schools widely vary in their official uniforms, and not only do these uniforms have official seasonal variations for summer and winter conditions. In addition to this, schools without iron fisted discipline levels see schoolgirls modify their outfits by shortening the skirt (temporarily or permanently), wearing loose socks, wearing knee-length socks, and so forth. Boys may go as casual as they can in uniforms and not have everything fully buttoned and so forth.
School-assigned footwear is usually in the penny loafer style; that is, slip-on shoes. This is convenient in Japan where you would be taking your shoes off every time you enter a home.
In addition, and I realize this is stating the obvious, skirts are a lot shorter today compared to the 1920’s. WWII has come and gone and people want to enjoy their youth rather than feel like they’re serving in the military.
Cultural Power And Legacy
Sailor outfits are symbols of what was, to most adult Japanese people, a more innocent time, a time when the pressures of the modern world were confined to studying and when the rest wasn’t so bad.
Of course, the shorter the skirts get, the more this impression of innocence becomes associated with naughtiness and sexuality. This spin on things may be punted into the stratosphere by anime and visual novel games, but it is hardly an invention of fiction; it reflects a part of contemporary society.
Also, in modern times, 99%+ of Japanese public schools are co-ed. Therefore, boys are constantly seeing girls in sailor fuku, not only at a young age, but when there is, ah, shall we say, more to stare at.
Co-ed as the schools may be, groups teens of different genders rarely mix much (i.e. they aren’t seen going around town like that very often). There are various reasons for this, but keep in mind that public displays of affection are a much bigger no-no in Japanese society than in America. Such things are funneled into dating; this makes dating more intimate, but also keeps such intimacy from butting into the “buddy” system around which cliques form.
Thus, many boys have long memories of seeing girls in “innocent,” teasing sailor fuku but being only able to look, not to touch.
I think that this explains much about how adults look back at their school lives, about “what could have been,” and how these thoughts influence fiction in novels, anime, manga, games, and broader culture.