Which version are you doing? The winter or summer version? I can't remember but she has a different uniform for junior high and high school I think. I'm guessing you might want something with the long sleeves, since next con is going to be in October, but I'll put up some details on both. Found this if you have money and want the easy way out, but I dunno how you feel about premade outfits:
In the summer version, she has a white or a really light gray, short sleeve top with side pockets, which may or may not be functional. The collar, I dunno what they're called, is a charcoal color same as her skirt, and it has a single red trim on it. The bow looks red as well. Four black buttons on the front and it should be good to go. I think knee length white socks and dress shoes and maybe a brown, Japanese style backpack would be all that's needed.
I can vaguely remember seeing her in this too, but can only find a cosplay pic of it. The outfit has a gray vest over the shirt, with white buttons. Just another look I guess.
If you can make the outfit, this doesn't seem to be too hard. For the top, all you'd need to do is add pocket flaps and the buttons, find or make the skirt in that color and if you know how to, can make the collar part from that same material and color. The bow should be easy to do or buy. Her socks has a trim
The long sleeve version is basically the same but with longer sleeves. I've seen pics with and without two buttons on the sides of the sleeve tips.
Hope any of this helps.