The school may have this 'policy'... because otherwise, if they let 1 parent 'chaperone'....then they have to let ALL Parents be there too... and that, is another 'issue' all together.
The 1st dance for a child, is indeed a 'milestone' and I can understand how you feel.
Why don't you simply ask the school 'why' a parent is not allowed to be there? Perhaps, the school has incurred problems previously, whereby it is simply not conducive to it now. Or, perhaps, the kids feel, especially at this age, that having Parents there makes them feel like "babies."
I have no idea about this school's reasons...but just ask them. And ask them, as any Parent would, what the 'rules' are at the school dance: (ie: what is the schedule of it, the entertainment, the code of dress, the security/supervision.... what happens when a child goes to the bathroom... will they be alone or accompanied. What safeguards are there to ensure their safety, what entry/exit doors are unlocked or locked to ensure the safety of the children and to make sure all is accounted for, etc. etc.)
I don't think, a child's attendance at a school dance should be determined upon whether or not their Parent is there. Keep in mind, that at this age, they are self-conscious....and if all her friends are going, she'd want to attend too. Otherwise, she will have to 'explain' to her friends 'why' she can't go because her Mom can't. And this may make her feel awkward, if not 'weird' about it in comparison to her friends. Or her friends may tease her about it.
All in all... a 1st dance is very important at this age. Let her enjoy it. But just talk with her about it, let her ask questions openly, without judgment, and let her know what your expectations of her are too as a 'responsible' young girl. And most of all, let her know that you TRUST her. Keep an open dialogue about this and everything, so that you will always know what her feelings are and what is going on in her life... especially at this age. So that she will KNOW that she can come to you for ANYTHING anytime, good or bad. You want to stay in the loop. And this is one way to do it... when you can't be right there among her as a chaperone.
All the best,
Susan