What's a shame is that there was some need to report her for expressing an opinion. Even back when proms cost mere hundreds of dollars when everything was added up, just about every school I've worked at handled it poorly. You can't stop parents from being jerks or daddies from wanting their little girl to be the prettiest most special young lady. What schools could do is promote an alternative for expensive prom decisions, like instead take up a collection to spend "prom savings" on something useful like a dinner for homeless veterans or some such charity.
Keep in mind that very often it's the same parents who fund these obscene choices that will complain that they can't afford to pay for their young 'uns college educations.