Bat Mitzvah Dressing

Hello, i have a problem. I hope you guys can help me.
I have a friend that invited me to a bat mitzvah and i have no idea what to wear. I have a really nice jean skirt and a really formal shirt on top. Couls i wear that?
I don`t wanna come too over dressed or too underdressed. Many friends are wearing dresses but i don`t have any. What should i do and wear?
I hope you could help me.
I really need your help.