I have felt the same when I was asked to teach math. I turned that down because I felt I would be doing a disservice to the students. I struggled through Native Studies, however, as it was close to one of my teachables, History. I am glad I did, though, because I love teaching it and am now becoming qualified in it. Issues like this come up to see how we handle them, and may, in fact, bring exciting new opportunities.
You’re lucky you are on practicum, as there are associate teachers in place to assist you. Practicum is meant to throw you in to see what you’re able to do. You will NOT be penalized or seen as incompetent if you ask for help. The phrase at my teachers’ college was “teachers helping teachers,” and I see it happen every day. We need to ask for help; we cannot do it alone, especially when teaching courses we are unfamiliar with, which, unfortunately, happens regularly.
Good luck! Believe in yourself!
Teaching In Real Life 