I'm willing to bet it's because the possibility of a huge number of girls wearing the bright, flashy Lily prints is overwhelming. That really could be a disaster, and maybe it's easier to just ban distracting, large prints.
If they didn't want designer stuff, there are WAY pricier things than Lily that they could ban. I honestly think this is just because of the loud print factor.
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It is all 33girl's fault. ~DrPhil