No, "because" doesn't need a comma either before nor after.
Here is the breakdown:
1. We have the simple core sentence (called independent clause) "the girl suffered". This is the gist of the matter.
(An independent clause is a group of words that can stand on its own as a sentence: it has a subject, a verb, and is a complete thought.)
2. We have the dependent clause "because her grades dropped". It answers the question "why did she suffer?"
(A dependent clause is a group of words that also contains a subject and a verb, but it is not a complete thought. Because it is not a complete thought, a dependent clause cannot stand on its own as a sentence; it is dependent on being attached to an independent clause to form a sentence. You could never write just "Because her grades dropped" and finish there. One would ask: "And then? What happened because her grades dropped?")
Dependent clauses can often be identified by words called "dependent markers", which are usually subordinating conjunctions. If a clause begins with one of these words, it is dependent and needs to be attached to an independent clause. (Common dependent markers: after, as, although, because, before, even though, if, once, rather than, since, that, though, unless, until, when, whenever, whereas, while, among others.)
And there shouldn't be a comma. The sentences are joined by these markers, no need for a comma.
The reason you're confused is that in this case, just after "because"
3. there is another, subordinate sentence which should be between two commas (or two dashes or in parenthesis) to separate it from the primary sentence:
At the time the girl was working to pay off her parents' debt
But these commas have nothing to do with "because".
(We use a comma to enclose non-defining relative clauses and other non-essential details and comments. The comma is placed on either side of the insertion.
A nonessential element is a word, phrase, or clause that is not needed to complete a sentence. In other words, it can be removed and the sentence still makes sense and is grammatically correct. If removing the element changes the meaning of the sentence, it is essential. Nonessential elements need to be offset with commas, both before and after.
China, one of the most powerful nations on Earth, has a huge population.
W.A.Mozart, who lived in the 18th century, was a great composer
Cats, unlike dogs, can clean themselves.
My friend, Jim, likes to go scuba diving.)
Now that we deconstructed everything, this is the correct way to write your sentence:
The girl suffered because, when she was working to pay off her parents' debt, her grades declined.
The sentence between commas could be taken off and the rest of it would stand alone perfectly well. So the secondary sentence is an explanation of when (and why), something extra to the main sentence.
It could also be put at the end.
The girl suffered because her grades declined when she was working to pay off her parents' debt.
It could also be put at the beginning.
At the time the girl was working to pay off her parents' debt, she was suffering because her grades declined.
As you notice, the main sentence "The girl suffered because her grades declined" is without any comma either before or after the "because".
All this could be written as a conversation between a person of few words (one of those infuriatiing people, usually guys) and someone with a healthy curiosity.
A: The girl suffered.
B. Oh, poor her, why?
A. Because her grades declined.
B. I see! But why did her grades decline? Couldn't she study more?
A. She was working.
B. Why on earth would a student work?
A. It was to pay off her parents' debt.
You know when we would need a comma before "because"?
If it introduced a non-essential subordiate clause:
Unfortunately, because I woke up late this morning, I was a few minutes late to class.
Now, "Unfortunately, I was a few minutes late to class" is the main sentence (independent clause) that can stand well on its own.
(A comma is needed after “unfortunately” since it is a transitional / introductory word at the beginning of the sentence)
"Because I woke up late this morning" is a useful explanation, but not essential. Therefore it must be separated by commas, hyphens or put inside a parenthesis. It is a subordinate clause (a type of introductory element) at the beginning of the sentence.
If you are still confused, this is a useful resource