What could become policy is a once-a-semester “Jubilee” for test grades, without any price attached.
In the Bible, a Jubilee is a year of freedom, proclaimed every seven years (or every 49 years). In the year of Jubilee, slaves were set free, debts were forgiven, and the land was allowed to lie fallow for a season (Leviticus 25). The Jubilee was basically a “do-over,” a re-boot of the system of obligation and burden, when it became too crushing. It mimicked God’s grace, granted for no reason other than to let us breathe again, and give us a better chance in life.
I agree with Principal Shepherd that it’s wrong for one bad test, on one bad day, to ruin a semester’s worth of hard work. But instead of making students “‘pay for it’” further, declare a Jubilee, as some teachers do already. Throw out one test grade; or grant a gift of 10 points on one test, one time — just because. They’re decent kids; give them a little grace.