You are guessing and wrong. The story is easy to check. Holderness Academy requires a plain black, pleated, knee length tailored skirt with plain black tights. The Asda skirt meets the definition completely. However, the school specifies that the skirt must be purchased from their official uniform supplier.
The parents say this is about size rather than price. The smallest size available from the official supplier is 24in waist.
However, as per my previous post, the academy appears to be acting unlawfully. This skirt is clearly a generic item. Parents should therefore be free to purchase it from any retailer they want. They should not be forced to pay over the odds for a skirt from the school's official supplier.