Although you're right about the suit in SCIV, it isn't always like that.
Look at this SCIII discarded design:
http://soulcalibur.wikia.com/wiki/File:Concept21TAKISCIII.jpg
The socks are just traditional ninja garb. Even Taki's usual full body suit is pretty much just using their imagery considering the sandals there, even if there aren't socks. If it was really meant to be school girl socks, she'd have shoes of some kind, not the traditional ninja sandals and socks.
I just don't think the imagery of a sailor suit can be "suggested" without even any kind of skirt, sailor collar or the characteristic colors. Even if your points were right, the only similarities you've mentioned to a schoolgirl outfit are socks and a fake skirt. That in no way automatically means "school girl".
If they really wanted a "sailor suit"-like outfit, they could easily have closed the armor, making it look like a skirt, and included some kind of colar, but there's nothing like that. And, like I said, the divisions of the armor are even orientated in the wrong way to match a schoolgirl skirt, they're just standard plated pieces (in addition, making plainly visible shorts there completely defeat the point of a "skirt").
They clearly want to give her an "youthful girl" image, but that doesn't mean school girl, if there's no actual association with the female school uniform design.