This is something I hear a lot, “But isn’t it wrong to start a sentence with a conjunction?” (When I read it, there is no conjunction at the beginning of that sentence.) And I always reply, “You learned that in grammar school, didn’t you?” And they always respond in the affirmative.
I don’t know what these teachers are thinking, but I have a guess. Before we get to that, however, remember that grammar school teachers say a lot of things that are wrong. I believe that the rule against starting sentences with a conjunction is to avoid writing like this:
Don’t write like that. But don’t worry about starting a sentence with a conjunction, either.
 Much better is the creepily cheery poem: