creating text marks and using numeric values too

I would l like to have an option with my grade entries to have not only numeric entries, but also ones for absent, excused and missing. The first two I would like the grade to not be calculated, however the missing assignment, would be calculated as zero. How would I set this up?

Hi there,
In the current version, the only way for a cell not to be taken into account is to be actually empty, any icon or text will trigger the calculation in that cell.

If, for example, you have a numeric grade in a column and you type 'missing' or any other text in the cell, it will be calculated as zero.
We'll try to offer more options in this regard in our next major update