hmmm had this problem myself a few months ago (post baby weight is joyous!) lecturing is pretty professional so the safest option is a suit IMO. Can you borrow one from someone? (what I did) or if all else fails have you got a really smart skirt/ trousers and smart blouse/shirt.
I am a great believer in 'you can never look too smart' so I always overdress rather than underdress iykwim?!?!