At school, I was always told “You never start a sentence with because”. However, now I see sentences like “Because of increasd pollution, there will be global warming”, and “Because the dispute couldn’t be settled, people lost their jobs”. Is it gramatically correct to say this?
Perfectly corect. The teacher in school was trying to get you to write more, so she banned the use (that’s the kindly explanation; the other is that she never bothered to learn the rule and was merely repeating what her teacher told her erroneously).
But there is no rule of grammar that prevents you from using it.
I’d usually begin such a sentence with Since… (in the case of accepted bland fact or Given that… in the case of something not proven, but that I would like to accept as true for the purposes of the statement or As a result of… in the case of a direct causal link.
That’s just me though; I’m not sure if that is the best possible practice.
This is another one of those rules that really should be worded, “Unless you know how to do it grammatically, never…”
This reminds me of a paper I wrote in college once. It came back with one red-ink note from my professor, and that on the last page. It pointed to a circled few words in the middle of a paragraph and read, “Technically, this is a comma splice, but it’s so beautifully done that I can’t mark off for it. I had to read it three times even to convince myself it was ‘wrong.’”
I changed my major from Drama to English later that week, if memory serves. 
The publication I work for frowns on using “since” or “as” to mean “because”, because they can be taken to imply a time aspect (since=in the time that has elapsed from; as=while). That’s just a house style thing, though.
I see no grammatical reason not to start a sentence with “Because”. It’s often not the most elegant or “active” way of writing, though – to replace the original examples, I would consider “Increased pollution will lead to global warming” for the first and “The dispute could not be settled, so people lost their jobs” or “People lost their jobs because the dispute could not be settled” for the second, depending on the emphasis required.
In the case of prepositional phrases (your first example) and adverbial clauses (your second example), beginning sentences with “Because…” is completely acceptable. The problem arises with fragments that young students seem so fond of using, such as:
“Because it’s green.” or “Because the President declared war.”
These are quite unacceptable. They are not complete sentences.
Often teachers will make the blanket statement of not using “Because…” at the beginning of a sentence to keep students from using fragments and hope that later when prepositional phrases and adverbial clauses are introduced, the student will see the difference.
TV
Ah yes, thanks TV time - that’s it! I knew I vaguely remembered my teacher banning sentences starting “Because…” but couldn’t work out why. I guess it’s just that (nowadays!) I’d never dream of coming out with a sentence such as “Because it’s green” but at school, with question papers asking “Why…?”, it seemed natural to answer “Because…”
Whereas the correct response is obviously “Just because.”
