Because is not a non-finite verb; it is not any type of verb. It is a conjunction. Many style manuals (and teachers who were brought up on them) say that you should never begin a sentence with a conjunction (because, and, but, since, etc.). English writers, editors and proofreaders have been ignoring this "rule" for hundreds of years. As a teacher, I never correct this usage in my students' writing unless they overuse it.
I recommend that your daughter not argue with her teacher. That argument can't possibly have a favorable outcome.