When the difference in expectations involves the faculty member teaching the course, there is a formal process to resolve the differences. This "formal process" is also just good advice on how to resolve differences in many situations. I recommend approaching all these steps from a place of respect for each other as humans; that is, we make mistakes and are often juggling a lot professionally and personally.
Please follow the following steps:
Review the syllabus and all course communications regarding the particular issue.
Identify where you believe an error was made and gather all the documentation.
Organize your thoughts.
Schedule an appointment with your professor.
Meet and discuss with your professor. Try to resolve your differences, together.
This is the most important action you can take--to work things out, together.
If this conversation doesn't resolve the differences and you maintain the professor made an error, then your next step is to make an appointment with the director of your School. If you don't know who this is, look on your School's website.
Bring all the same documentation and notes from your meeting with the professor.
If this conversation doesn't resolve the differences and you maintain the professor made an error, then your next step is to meet with me, your assistant dean.
My role is not to resolve the differences, so you don't need provide me all the documents and arguments. I will listen to an overview and then recommend next steps.
You are welcome to skip meeting with me and/or not wait to meet with me before the next step, which is to meet with Ombudsman to file a formal grievance.
A formal grievance is kind of like filing a lawsuit where you will have to present your side and the professor will present their side and an independent committee makes a decision. Read more about the process and how to start it, at the Ombudsman's website here.