Focus on neutral colors for camisoles (5) tank tops (2), cardigans (5), vnecks (8), leggings (5), jeans (3), and pants (3). Have a few pencil skirts in solid neutral colors, or have a patterned neutral skirt. Have lots of regular tshirts for lazy days.
I think having shirts/jeans/leggings are most essential for school clothes, and it's good to have a few sweaters too. Skirts/dresses can also be pretty important if you like wearing them, if you don't then you don't need a lot of them.