Welp, not a lot of choices that can be done in a week..The grunt that Azura said is probably the fastest. Since you only need a shirt and pants and black shoes..That's all super easy..you can have whatever hair you want..Then buy a cap that they wear, in fact you can have mine if you do that (if you plan on taking that advice, I'll give you a list of what I'm cosplaying so you can find me).
Otherwize, I would say go for things like normal school uniforms, or L from death note, in which you will still need a wig which you may not get in time depending where you buy from.
There aren't many cosplays that can easily be made in the week that you have left..
Cosplay DA Regular DA
-Tsuruya-san (Haruhi Suzumiya)
-Kagami Hiiragi (Lucky Star)
-Minimandy (Grim Tales)
-Mimi (Grim Tales)
-Yui Hirasawa (K-On!)