I'm going to shamelessly crib this from a previous thread where Gothicmama wanted a capsule work wardrobe for £50 - I think it all still applies but if you have the budget get more multiples of each item or add extras.
Might also get knitwear instead of the jacket.
If primary school make sure everything is machine washable to counter paint, sand, snot, glue etc.
one pair footwear (boots/shoes as you like but keep them polished and looked after)
one smartish jacket (not part of suit)
two smartish trousers
scarf/wrap thing
four or five tops (sleeve length will depend on aircon, heating etc - you can always leave jacket and/or scarf on if it's icy)
I don't wear black (and am evangelical about getting others to stop, it's so unimaginative), so if this was me picking from clothes I've actually got I'd do:
tan low-heeled boots
cream linen/cotton jacket
one pair chocolate brown trous
one pair tan trous
cotton tops with flattering necklines in
brown s/s
brown l/s
terracotta 3/4 s
olive green 3/4 s
one cream blouse in case need to be v formal
orange patterned scarf
but there's nothing to stop you getting black shoes/trous/jacket/scarf and coloured tops, or whatever colour combo you like.
To get this for £50 and have it all fit and last is a bit beyond me tbh - I'd normally spend that much on just the boots and make them last five years . If you go with a strict list and ferret through sale rails you may find some bargains though, and places like Primark, Peacocks, Matalan etc are good for cotton tops and blouses, and you might get lucky with trousers and jackets if you're the right shape.
I really wouldn't skimp on footwear though - non-leather boots with cheap heels never seem to last that well, and they've got to be comfy if you're running round all day.
Don't forget nice bag you can keep all your stuff in - armfuls of papers dropping everywhere never a good look. And if it matches your shoes so much the better.
I'd also add some statementy jewellery - a big pendant (shell, wood, amber etc) that matches your trousers is a v easy way to make plain top and trousers more interesting.
I deliberately haven't mentioned skirts as I find trousers more useful - and skirts often demand a second pair of shoes which can take a sizeable chunk out of the budget, but if you have a suitable pair already a skirt gives a whole different look for little outlay.