She didn't go to class today, because she was sick.
In this sentence, there is a comma before the because, so can I assume the clause "because she was sick" is a independent one, and "because" here is a co-ordinate conjunction.
Is't is wrong using the comma there?
Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.
*** Common starter words for introductory clauses that should be followed by a comma include after, although, as, because, if, since, when, while.While I was eating, the cat scratched at the door.However, don't put a comma after the main clause when a dependent (subordinate) clause follows it (except for cases of extreme contrast).
Because her alarm clock was broken, she was late for class.
If you are ill, you ought to see a doctor.
When the snow stops falling, we'll shovel the driveway.
1. She was late for class, because her alarm clock was broken. (incorrect)
2. The cat scratched at the door, while I was eating. (incorrect)
3. She was still quite upset, although she had won the Oscar. (correct: extreme contrast)
There is a difference in meaning between these two sentences:
 She didn't go to class today, because she was sick.
=> She was sick.
 She didn't go to class today because she was sick.
=> She wasn't sick. She is absent from school today because of some other reason; e.g., maybe she has gone on holiday for a few days.
If the verb in the main clause is negated (i.e., didn't go), it will also negate the the verb in the subordinate clause (i.e., was sick becomes wasn't sick) if there isn't a comma to stop it.