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. 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 under that collar, trust me. 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.
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. 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.
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.
The most common color by and large is navy with white trim, but green, gray, and beige are common.
I didn't touch on the more esoteric or foreign styles of uniform (like the ones in Eva, or any you'd more likely find in a Catholic private school) because they're WAY easier to find references for and I didn't feel it was necessary at the time. 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 that 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!