In a "calculated" column, you can specify a mathematical formula to derive values from another column (or columns). eg (Q1 + Q2) * 0.5
In a "manual" column you'll be able to enter all grades in manually, or copy values from another column and then alter them manually.
That's exactly my idea: two final columns (in califications table) with the same calculated values: the first not editable (calculated), the second (manual) with the same values of "calculated column" editable by the teacher (just in case he want to modify this value), and a save button in the end of the table that updates the final califications of students.
Chhers