I know this is resolved, still....
just a suggetion use a look up table or create another cte for all grades.
Then join this look up table with your main cte (or table) where score in main table is between score range in the grade lookup. this will give you more felxibility.
with sample as (
select 6 as Items,0 as score union all
select 2 as Items,30 as score union all
select 10 as Items,50 as score union all
select 12 as Items,75 as score )
LkpGrade as (
select 'Grade1' as Grade,0 as MinScore, 39 as MaxScore union all
select 'Grade2' as Grade,40 as MinScore, 69 as MaxScore union all
select 'Grade3' as Grade,70 as MinScore, 100 as MaxScore
select g.Grade, sum(s.Items) from sample s
inner join LkpGrade g on s.score between g.MinScore and g.MaxScore
group by g.Grade;
hope this helps