What should I wear for a potentially awkward event?

ok, my boyfriend and I are going out to dinner, just before christmas with someone he gets along with. He is the polar opposite of me, ie he never gets flustered and he is really self ured. The person we are going to meet is one of our ex teachers and she never liked me and the thing going through my head atm, typically, is what on earth am I going to wear. I don't think it is going to be overly formal, but I will be nervous enough as it is without feeling uncomfortable in what I am wearing, and I am the sort of person who is really self conscious. Any suggestions on what basically I could put together, ie trousers, dress etc? please