Not sure what the issue is here. Why not just put "union all" between your two queries, and move the ORDER BY clause to the end?
SELECT GRADE_TYPE_CODE, ROUND(TO_CHAR(NUMERIC_GRADE),2) AS GRADE
FROM GRADE
WHERE STUDENT_ID = 10 AND SECTION_ID = 5
union all
SELECT 'Average is:' AS GRADE_TYPE_CODE,AVG(GRADE) AS GRADE
FROM
(
SELECT AVG(NUMERIC_GRADE) AS GRADE
FROM GRADE
WHERE STUDENT_ID = 10 AND SECTION_ID = 5
GROUP BY GRADE_TYPE_CODE
) myInnerTable
order by 1 desc, 2 desc