C1 : 85
C3 : 70
C2 : 67

SELECT
ccode,
ROUND((passed::numeric(15,2) / taken_course::numeric(15,2)) * 100, 0) AS percentage_passed
FROM(
    SELECT
    ccode,
    sum(CASE WHEN grade > 2 THEN 1 ELSE 0 END) AS passed,
    count(1) AS taken_course
    FROM
    hasStudied
    GROUP BY ccode
    ) foo
ORDER BY ccode
-- since you want to order DESC by values, instead do
-- ORDER BY percentage_passed

create table #temp(StId int, ccode varchar(5), grade varchar(1))
insert into #temp Values (1,'A1','A'),(1,'A1','F'),(2,'A2','B'),(3,'A2','F'),(4,'A2','F'),(4,'A3','F'),(5,'A3','F')

;with cte as (
    select ccode
    from #temp
    group by ccode
)
select cte.ccode,ratioOfPass = cast(sum(case when t.grade <> 'F' then 1.0 else 0.0 end) as float) / count(*)
from cte
inner join #temp t on t.ccode = cte.ccode
group by cte.ccode