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A school uniform signals purity, purposefulness, and properness. The witch outfit is intentionally too wild. As usual, the magic kingdom is the unconscious 'otherside', so it makes sense that the uniform there is adventurous and breaks the rules of proper dressing to fulfill a multitude of fantasies. Waitress-like skirt, corset (or a corset-like vest?), puffed sleeves, cuffs that remind of Peter Pan, long black socks, Art Nouveau, bow tie, check-patterns, suspenders, you name it. Girls that age explore their possibilities between cute, sexy, and graceful, and the uniform works as metaphor for that. Some kind of "rpg practicality" is also implied by the design, suggesting some dream job thematic. Also note the colors: reddish purple, pink, and yellow. Pink for girliness, yellow for exploration. Maybe the purple-red is just a cross between red for passion and purple purple for nobility? In magic world their socks are black, and in this one they are white.
For what it is, the magic school uniform is a good design, but it decidedly does not fulfill the role of the school uniform. Conceptually it is part of the magical girl wardrobe. This is confirmed by how the magic school functions in the story; it's not very school-like.