This is an old guide I made for drawing seifuku. I happen to own one I got as a kid directly from Japan, so it is still mostly accurate.
I will mention some things though:
The tie that came with mine is a gigantic triangle. There were no instructions that came with it, but I think I'm incorrect here and you don't roll it (but you can if you don't want the third flap visible at the back). The site I bought it from assumes purchasers are actual students, I believe. There's probably variations from school to school, and in anime I've seen a lot of wacky tie designs. I've used normal neckties and ascots with my uniform, anything will fit. Not all uniforms come with the tie band, it was an optional feature on mine.
You should also know that the skirt drawn here is inaccurate because I stuck the pocket midway between pleats; the pocket opens directly from the seam/pleat fold and is completely invisible from the outside. It's actually incredibly roomy, and for some reason, points backwards (unless they put the tag in the front in Japan). This could be unique to mine. There's also only one pocket on mine.
Also remember there are two main types of pleats: the kind that's folded one over the other infinitely, or the kind that doesn't overlap at all, like a traditional kilt. Pick the kind you want and remain consistent to that design, if you're doing a serial manga or somesuch. Skirts shouldn't suddenly change their engineering between panels. They can, however, loose the pleats at the bottom over time, which can be fixed via ironing.
The material of the shirt is a stiff, starchy sort of cotton, and the skirt is rather rough and heavy, possibly wool. The tie is a light silk/satin. The stripes on the collar and sleeves are sewn on, so if you do an extreme close-up, you may want to add the thread for realism. All the materials are washable. Also, this may just be due to my particular tailor, but it is really high quality and I've never lost a stitch in the 10+ years I've owned it! Then again, if I wore it every day like I would if I had been in school, it might've worn out.
Summer uniforms are the more popular lighter-colored ones with shorter sleeves and skirts. In the winter they switch over to the longer sleeves and hemlines and typically make the entire blouse out of the collar color. This may or may not be a universal standard, I really don't know, but that's what I've observed in anime and photographs. (While anime is fiction, remember it's created BY people of that culture, who are intimately familiar with its trappings and idiosyncrasies.)
The most common color is navy with white trim, but green, gray, and beige are also common.
I didn't touch on the more esoteric or non-Japanese styles of uniform (like the ones in Eva, or any you'd more likely find in a Catholic private school). But if you're setting your picture in another country, you should do some research to see what their uniforms are like, they can vary quite a lot.
If your setting is a fantastic one then take this with a grain of salt, it's your world, go nuts.
Considering the wild popularity of this piece and some thefts I've regretfully had to report, I feel the need to point this out.
You may use this AS A REFERENCE ONLY.
You may NOT RE-USE, RE-DISTRIBUTE, OR ADAPT THIS INTO YOUR OWN TUTORIAL/GUIDE/REFERENCE/WHATEVER.
If I find this used, traced or copied in such a manner I will report without warning or hesitation. This makes me very sad, so please don't make me do this anymore!