My daughter had a little issue at that age (and still kind of does) where she only wanted to wear dresses. (No pants, shorts, tights, jeans, etc.) I did some serious thrift and second-hand store shopping because it's actually hard to find comfy cotton dresses in retail stores. I have quite a few 'cooler weather' dresses that she's outgrown and we'll be getting rid of, but for this long summer, try some tank style ones. I got lucky at Old Navy, Land's End, Kohl's, or give ebay a try. Disney store has some cute one-piece ones that might already be cleared out by now. Her favorite shoes are kind of a croc style that we found at Children's place for 7 bucks. The other thing that worked for me, back when she wouldn't even get out of her nightgown, was the bribe factor. I would suggest we go somewhere that I knew she loved like McD's playland or the library, but then we need to get dressed. If she refused, oh well, guess we won't go then. I think once she gives up that power struggle (and thinks it's her own doing) she'll come around. And I'm sure, being a teacher, how to limit your negotiating and maybe give her 2 choices max. (I actually teach K-5 music, too.)
Good luck!