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  How do you ensure that a test does not lower a student's average?

It should be indicated that the test is optional.
To do this, in the creation/modification window of the tests, tick Optional test and choose if the grade should count:

• as a bonus : only points above 10 are taken into account in the calculation of the average. For example, a score of 15, 5 will be added in the calculation of the average. Scores below 10 are shown in gray and in parentheses.
• as a grade : the test is taken into account only if it improves the student's average.
• according to the default threshold : only tests with a grade above the threshold grade are taken into account.