Your elementary school teacher either was not very conversant in English grammar or deliberately intended to mislead you. But now that you're in the right place, I'm positive you'll slowly unlearn all wrong ideas stuffed in your mind by incompetent trainers like this one.
As you might be aware of, the word ''because'' belongs to a group of grammar terms in English language known as subordinating conjunctions. Subordinating conjunctions are words or phrases that join an independent clause (a group of words that has a subject and a verb and expresses a complete idea) to a dependent clause (a group of words that has a subject and a verb but doesn't express a complete idea) to form a compex sentence. The function of a subordinating conjunction is basically twofold: To provide the necessary transition between ideas in the sentence and to reduce the importance of one idea so that the reader understands which between the two is more important than the other.
But you ask if it's correct to start a sentence with the subordinating conjunction ''because.'' The answer to that question is absolutely YES. The only convention to follow in such usage is to put a comma at the end of a dependent clause if ''because'' introduces a complex sentence. On the other hand, comma is uncalled for if the independent clause comes before the ''because'' dependent clause. So ''Because I'm ill, I failed to attend the meeting yesterday,'' but ''I failed to attend the meeting yesterday because I'm ill.'' Important is to note that the rule applies not only to ''because'' but to all other types of subordinating conjunctions.
I hope I've helped in some way to allay your confusion. Partially prepared teachers can really create very serious problems for their students.
