Wedding attire for teenage girl

[sybilmc]

Can anyone tell me what a tall 15 year old girl might wear to a wedding at the end of March this year? I am struggling to find something nice (and appropriate).

Knee length dress with a cardi? Flat but dressy shoes? What's her style?
Black trousers and a smart top and cardi would work too if she doesn't like dresses.
I'd look on ebay for smaller size adult formal stuff. People seem to buy formal dresses, wear them once, then get too fat and sell them, so there are bargains to be had!

I have just bought my teen a black floral dress from Jack Wills it looks fabulous!!
www.jackwills.com/springbourne-ditsy-print-dress-100014536002.html?cgid=Ladies_Dresses_Playsuits#start=7

Thank you!