Excell - Getting the average without taking into account zero and DIV/0

Hi,
I posted this about a week ago in a German forum and so far havent received any answers to my question. So lets find our if you guys can help me which would be very appreciated.
I have put all my students test results into the excel form attached and have put all necessary formulas into the table so that I (and the students) can see very easily who is doing fine and who does need some extra grinds. The problem is that from C29:C49 I try to have the table calculate the average of all results for each individual student. All together we will have about thirty tests this year so that since we dont have all these results yet i have a lot of zeros (the student didnt take the test) or DIV/O results (nobody has sat the test so far) in some of these boxes. When I try to have the table calculate the average for each student (in per cent) (with student no 1 this would be No.1 D29:AG29) it doesnt work because of the zeros and the div/o results. Does anyone know the formula I have to put into C29:C49 so that it'll work? If you can help this would be very much appreciated and if you ever happen to be in freiburg, germany I#ll take you to the beergarden and the beer is on me.
Vielen Dank und Grüße
Daniel