What does one wear to a bat mitzvah?

I'm going to a bat mitzvah for my Jewish friend in a couple of weeks and I'm not sure what to wear. The ceremony is in a hotel with a party immediately following. I am fifteen. I have a flowered dress that goes to probably about three inches below my knee. It is not low or tight or anything like that, but it is strapless. I was planning to wear a cardigan over top, but I wasn't sure if the dress would be appropriate or not. I have never been to a bat mitzvah before, so thank you in advance for your help.