Education Law Questions? Ask an Education Lawyer
Hi. First, my personal opinion on the subject is that grades are particularly arbitrary in most curriculum. If you think something is wrong with your grade and it was unfair, the best is to go see the dean of the university and explain the situation. Most university have a revision procedure (with an associated cost).
To add to the answer offered earlier from the other expert, my thoughts and suggestions would first start with speaking to the professor that you had the course with and see where the on-line discussions were lowered and why. Ask that you would like a private meeting over the phone at the professor's best time to speak and ask what has what it seems to be a lower score for on-line discussion. See if there is anything that can be added to the work done by yourself to raise the grade to make it an "A". Map out what you want to ask the professor first so you go in with facts and reasons why the grade should be raised.
Best to speak to the professor first before going over his or her head as this can make the change of the grade that much harder.
If nothing can be done working with the professor, then see what your school allows in petition of your grade. Allowing a petition to be conducted by the department will allow you the chance to offer your reason why the grade should be raised.
Talking to the professor is fine before the grade are entered in the system, once they are there you need to go trought the revision procedure (also note that there is always a possibility that the result become lower than the original).
Speaking with reason and clarity why you believe that the on-line discussion was handled well by yourself in class will help in convincing the professor to hear your side of things and will possibly change your grade. If after grades are posted and there has not been any bump in grade, then by all means speak to administration at your college and see what recourse you have to petition the grade.
Speaking though to the professor first before going over their head would be the proper and professional way to handle things.