Grad Dresses!

So my Grade 8 grad is coming up, and I need a dress! I hate wearing dresses, and I am very picky! 
The dress has to go down past my knees, and it has to have 3/4 length sleeves (or almost 3/4). It has to be designed so that I can wear a bra with it (so only a low open back, no cut out shoulders). I will be wearing it in a church for part of the ceremony, so it has to be church appropriate! 
If you know any stores or websites where I can get dresses like that, please leave a comment below! 
Thank you! 
Note: I am fairly skinny. I wear about a size L in Girl's and XS-S in Women's.