AVERAGE help, using it on formula returns.

Hey all, basically I work in a school and have a massive list (some 1200 pupils) of grades (over 35 each) in an excel 2010 workbook.
For each grade, we have a lookup table returning a value for the grade, so we can work with the numbers in the sheet and generate reports.
ie, A = 52, B = 46, and so on.
Now what I need is the Average number for entire classes but I have hit a wall.

I have obviously used =AVERAGE(N5:N250) and half the time it works, but often it brings up a #N/A because a grade isn't present somewhere in the list of students. Because not every student gets a result grade in every subject.

Is there any average formula's i can use, that will bypass the N/A's and just count the number values being returned from my lookup table?

Currently its a case of copy/paste just the values into new columns or manually deleting the lookup formula from cells next to a no grade entry, but as I'm sure you can imagine, for all those students and all those classes its highly time consuming.

Any help would be massively appreciated :D