Alright! Here is my 2 cents! Looking at both the old design and your new one. (:
So I agree with Mango that the halter into the full back is odd and impractical. In the new design I'd say the long sleeves are also impractical if you want to use this as a real uniform. Long poofy sleeves like those would get into the food and stuff.
But I think the biggest problem is that you're sketching this as a dress, yet in reality it would be a dress with an apron. Aprons, usually don't have backs so I think that's what makes it look odd. (:
Here I sketched up a way to do it with an apron and a dress. (: I know you said you didn't need me to but I figured I would anyway. (: