Created a data set "DataSet1" as below
select 'Adam' AS Name ,'ABC' AS School UNION
select 'Priya' AS Name ,'BCD'AS School UNION
select 'Lucky' AS Name ,'ABC' AS School UNION
select 'Rama' AS Name ,'BCD' AS School
Added a tablix and group by School

Add the expression for Percentage column in the tablix as below
=(COUNT(Fields!Name.Value,
"Details")/COUNT(Fields!Name.Value,
"DataSet1"))*100
where "Details" is the group name and "DataSet1" us the data set name
Suresh
My Scribbling