Cómo calcular el porcentaje con una declaración SQL

El más eficiente (usando over()).

select Grade, count(*) * 100.0 / sum(count(*)) over()
from MyTable
group by Grade

Universal (cualquier versión de SQL).

select Grade, count(*) * 100.0 / (select count(*) from MyTable)
from MyTable
group by Grade;

Con CTE, el menos eficiente.

with t(Grade, GradeCount) 
as 
( 
    select Grade, count(*) 
    from MyTable
    group by Grade
)
select Grade, GradeCount * 100.0/(select sum(GradeCount) from t)
from t;