You don't cover the Blues... kudos to the young lady for graduating early and moving on to the Marines. Yes any young person who has earned the right to wear a Service uniform should be allowed to do so if they ask but if the answer is "no", suck it up and accept the "uniform of the day" or don't attend. We had a similar incident in our area just after my oldest graduated from boot camp. His take at the time was "the dude just walked off the Parade Deck as an f..ing MARINE and he's bent about high school shit...BFD!".
After seeing some of the suggestions regarding wearing the gown over the dress blues, I was suggest this, so I am glad you pointed it out first.
After due consideration, I have to agree that once asked and when the school admin stupidly responded the way they did, she should wear the gown for as little time as possible, and wear the blues the rest of the time.
As a side note, I've gotten some notes from my boy in San Diego - things are going well for him, boot is an eye-opener to be sure, especially at his age (26), and he is already seeing some of the issues that DIs and other leaders have to deal with, like fidgety recruits who are supposed to be still at attention. He should graduate in August - pictures then.