We have a chain consignment store for ladies clothes that i just discovered and it is AWESOME.
From your post, Ann Taylor is probably way out of budget. Target, or Kohls ( but generallyit's better if you have their store credit card and get in on the big deals that way)
I would say you need 3 pairs of Kakhi colored pants, and 3 pairs of black, navy, or brown.
And then 5 tops that you can mix and match with the bottoms, if you have a lavendar shirt you can wear it with the kakhi or the black.
I would stick with solid T shirts that have a fancier neckline, and long sleeve T type shirts. Because you'll be busy and it will probably be hot.
you can dress up the Tshirt with 2 beaded or chain necklaces. if the kids don't grab at you.
I would get a pair of black and a pair of brown loafer/athletic skeetcher ish shoes instead of sneakers just because i think it looks a little more professional, but if hte teachers wear sneaks then anything goes.
I would say you could get the T's for $7 apiece x5 around $35 and the pants for $15x4 around $60 and then $20 each for 2 shoes = $40 so under $150 for a weeks wardrobe.
Oh and when picking colors decide on 2 basic colors you look best in and build on that. so if you have red hair you might look good in a moss green and dark brown or something, If you are blonde, navy blue and pink, what ever usually you sort of know which colors to avoid, but if you really focus on getting 2 good colors for yourself you'll be all set.