I would own up and tell the class that I made a mistake! I believe when you teach you have to be honest and honesty involves holding your hand up and saying 'Sorry class, I got that wrong!'
After all, teachers are only human (no robots yet thank goodness!) and a class should be able to experience their teacher occasionally making a mistake - but not too many!
I also think that if a teacher doesn't know how to answer a question they should say so and tell the student that, for the moment, ignorance is bliss. Of course, a good teacher will do the necessary research and come back to that same question, that same student, and give them as comprehensive an answer as possible.
When teaching English I always try to be as open and honest as possible. On the rare occasion I falter, I simply tell the class that I do not know. Or have forgotten. It happens to us all at some time? That way, everyone knows just where they stand.