Sweet 16 Invitations

I am having a Sweet 16 for my daughter. I found the most beautiful invitations online that looked like tickets to the Royal ball. Instead of using the stub part as a Save the Date, I had them make them into RSVP. I was asked about the wording of the invitaiton and chose something like this:

It has been proclaimed that on the 20th day of August 2005, there will be a Royal Ball. Your Presence is Requested. Please join (hae name) in the celebration of her Sweet 16 at (name of hall) ("Address) *Dress is Formal.

That is the part that my mother is mortified with. She told me that it should never state that on an invitation. If they are invited to a Royal Ball, they will know that the dress is formal. I figured that many sweet 16`s are not formal, they are just parties and I wanted her friends to know to wear their prom dresses/gowns and to be formal. I do not want to let anyone in wearing jeans and sneakers! Is this proper?