Average grade calculator

This calculator automatically calculates the average grade for each person in the table when a grade is given, however the average can also be calculated by clicking the button below the table.
If an assignment is unsubmitted - the cell color changes to yellow.
If average grade is < 40% the Final Grade cell color changes to red.
If an invalid (non-digit) character is entered in editable cells the input is not reflected in the table.
The second table calculates how many assignments are due for each person listed in the table.
JSFiddle link: https://jsfiddle.net/cmulation/9wvm0tkf/