Can a sentence start with "Because"?

Solution 1:

It is appropriate when you have simply flipped the clauses:

I stayed inside because it was raining.

Because it was raining, I stayed inside.

What the teacher was trying to teach was that a a subordinate clause is not a complete sentence:

I stayed inside.

This is a complete sentence.

Because it was raining.

This is not; the conjunction because makes it subordinate and therefore, it requires an associated independent clause.

Solution 2:

Because I don't know your teacher, I can't know her motivations. However, there is nothing wrong with starting a sentence that way if you are careful to write a complete sentence.

Solution 3:

The rule is really made up to prevent primary school children splitting their sentences up randomly. This is just one of those rules that's made to be broken... when you know it sounds ok.

Solution 4:

Because you asked this question here, I'm obliged to say yes.

Solution 5:

Because you can, it doesn't mean you should...