What does it say when you try to change into the uniform?
I think that's happening because it's not supposed to be possible to equip the maid outfit before school (so I'd also be interested to know how you did so I can fix it!) but you should be able to go into the equipment screen through the pause menu and change Missy's clothes to the school uniform manually. Are you running version 0.8.8? I seem to remember fixing this exact problem a couple of updates ago and there are so many failsafes for it in there now, if so I'd be interested in seeing the save if you wouldn't mind emailing it