Calculating percentage of a changing total amount

Hi, Just a quick question that I'm hoping someone could help out with please-

I am wanting to keep track of student's hours of attendance as a percentage. Each class is 5 hours long and over the course there are a total of 12 classes. What I would like to do is have a formula so the percentage is calculated from the new total every week. It's a little hard to explain so an example -

class1 a student attends for 5/5hrs so the % cell would output 100% next week the student shows up for 4/5hrs so I want the percentage cell to output
90% if the 3rd class they don't show up at all i.e 0/5hr I want the % to show 60%

This seems easy enough but how do I make sure that every week the percentage is calculated by the new total amount of class time? The solution may be more simple than I think but I've tried a few things and haven't been able to work it out yet so that it all works properly. Any helpwould be much appreciated.

In you spreadsheet in Cell A1 enter Class Hours
Then in Cells B1, C1, D1, E1, etc.(or however many columns you need) enter the number of hours you want for each class, these will be entered at the completion of each class.
For this example B1:K1 are the number of classes you want to track.
Then in Cell L1 enter the following =Sum(B1:K1), this give the total hours possible
In Cell L2 enter the following formula: =SUM(B2:K2)/$I$1
This formula will be copied down for all of the Students.
In the above formula the $I$1 indicates you will always reference this cell for the total number of class hours.
In Cell A2, A3, A4, A5, etc enter the students name:
Then in each of the cells to the right of the students name (i.e. B2, C2, D2, etc) enter the hours of attendance, (as each class is completed).
In the above example format column L as Percentage and this should work for you. There might be an easier way to do it - but this might give you the most flexability - i.e you can vary your class hours as necessary.

Thanks heaps for that, I was way off with how I was trying to do it!