With the following MySQL table:
+-----------------------------+
+ id INT UNSIGNED +
+ name VARCHAR(100) +
+----------------------------
I've got a very very similar issue, that's why I won't ask the same question, but I will share here what did I do, I had to use also a group by, and order by AVG. There are students, with signatures and socore, and I had to rank them (in other words, I first calc the AVG, then order them in DESC, and then finally I needed to add the position (rank for me), So I did something Very similar as the best answer here, with a little changes that adjust to my problem):
I put finally the position (rank for me) column in the external SELECT
SET @rank=0;
SELECT @rank := @rank + 1 AS ranking, t.avg, t.name
FROM(SELECT avg(students_signatures.score) as avg, students.name as name
FROM alumnos_materia
JOIN (SELECT @rownum := 0) r
left JOIN students ON students.id=students_signatures.id_student
GROUP BY students.name order by avg DESC) t