Formula to pull summarized value?

I'm creating a grade book. I have a table that contains the scores of each graded item for all students, in all of my classes. This includes homework, quizzes, tests, final exam, labs, etc.. Each class may have a different set of graded items (some classes may not have a lab, for example). So in this table, I have a student ID, a class ID, the graded item category and the score. In a related table, I have a list of my students and on a layout I can summarize, using a portal, the total and average that each student has earned for each graded item category. No problem.

What I'd like to do is create a field in my student table that will hold each of these averages separately. Each student will have 5 or 6 fields that will hold averages for each category. Somehow I need to pick up these summary fields in a formula. I need these values so I can manipulate the numbers for calculating weighted averages and projections during the semester when some categories have not been completed.

Thanks in advance.