Error Message - Value 'Q' is not facet-valid with respect to enumeration '[A, B, C, D, F, EXC, DROP]'. It must be a value from the enumeration. The value 'Q' of element 'Final_letter_grade' is not valid.

<?xml version = "1.0" encoding="UTF-8" standalone="yes"?>
<!-- ?xml-stylesheet type="text/xsl" href="./theschema.xsl"? -->
<Class xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:noNamespaceSchemaLocation="aclass.xsd" class_number="301">
    <Student>
        <Name>
            <First_name>
                Joe
            </First_name>
            <Last_name>
                Java
            </Last_name>
        </Name>
        <Projects>
            <Project1>
                9
            </Project1>
            <Project2>
                18
            </Project2>
            <Project3>
                28
            </Project3>
            <Project4>
                31
            </Project4>
        </Projects>
        <Tests>
            <Mid-Term>
                82
            </Mid-Term>
        </Tests>
        <Final_grade>
            91
        </Final_grade>
        <Total>
            87.15
        </Total>
        <Final_letter_grade>Q</Final_letter_grade>
    </Student>
</Class>