I think you would be able to get away with cotton jean type trousers (i.e. not denim) as long as they´re not ripped or anything or a really skinny style. Otherwise a plain, comfortable pair of trousers. Pockets are handy especially at "that time of the month" as toilet breaks are often snatched wherever you can get one rather than a nice private comfortable 5 minute jaunt with your handbag.
Pumps or comfortable flat shoes, boots in winter especially if you´re spending time outdoors (at our school many TAs and teachers wear Ugg styles in winter, otherwise leather).
On top I´d say a T-shirt style / shape, either long or short sleeved. Classrooms aren´t usually cold but if you´re moving from room to room then a cardigan is wise.
Keep accessories cheap and washable. A watch is really useful for things like the patio or anywhere else you´re working without a clock.
At our school the TAs tend to do a lot of patio duty and helping with PE equipment (your role may be different) in which case you need a warm coat to pull on and off as you come in and out. Gloves, hat and scarf maybe too (or sunhat and sunglasses in summer).
Basically you want to be comfortable and to have clothes that can get dirty without creating emotional or financial strain for you. Also think about necklines and hemlines and general "decency" when bending over / stretching etc.
I would also recommend keeping a spare set of clothes, a top at least, at school in case you get wet, stained etc. (It happens!)