Combine two queries to one
That's not possible, because you have different granularity in the two queries. You would have to calculate one average by yourself with a formula like sum(grades) / count(distinct full-student-name). (btw, your use of distinct is not necessary, the group by does that already for you)
With this you would get the count correct, but you would sum too much grades, because they are multiplied by the grouping for classes. You can't do sum(distinct grades) because a student could have the same grade in different classes.
You also can't calculate the average of the averages. That's usually close, but still not correct.