Mermaid costume top ideas?

I'm considering being a mermaid for Halloween this year and I want to try to make a tail that looks kind of like this http://img.costumecraze.com/images/vendors/incharacter/1033-large.jpg but I'm not sure what to do for the top. I live in Michigan and it can be pretty cold on Halloween so I need a top that will keep me warm. The top also has to be school appropriate but I don't it to look little kiddish, I want to be a pretty mermaid lol. Please help me with ideas for tops. Thanks!