Teachers can exempt students from their Diagnostic. This is particularly useful if a student joins a class later in the year, or if the student belongs to multiple classes of a teacher. It is also useful for teachers who have classes with students at multiple grade levels.
To exempt a student from the Diagnostic, go to the Diagnostic Data page for your class's Diagnostic by clicking the Diagnostic Data button in the right-hand corner of the Diagnostic on your Assignments page. If the Diagnostic is not started, click Dashboard in the lower-right corner, then Diagnostic Data on the left-hand side.
On the Diagnostic Data page, scroll down to the Overall Performance by Student table at the bottom. Find your student, click the dropdown arrow for them in their Score column, and click "Exempt from Assessment."