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My 13 year old graduated last year. She wore a maxi dress with a slightly padded bust. I was concerned about too much padding and it being too inappropriate. I altered it to ensure it wasn't too low, added a statement necklace to cover up a little more skin and she wore a short shrug since she likes her arms covered. The school did send out a note saying nothing too short or inappropriate. Her graduation was held at night though and they went out for dinner afterwards. She has been able to wear her dress several times since and probably will again once the weather heats up.
My daughter is going to hers in a really beautiful slightly formal dress we got at Myer sz 14
I don't think it would need to be a formal dress or cocktail dress. Whatever she wears dress it up with some jewellery, nice shoes and a hair do.
Its not down to age really but personality. If she's girly then probably a skater style skirt with a higher neck or looser top. Colours, accessories and shoes is what ages it. Myers, kmart, pumpkin patch, asks, boohoo... Colour is key to keep it young.