Transact SQL :: Group By Count Once For Duplicate Column Values?

My data is like below: 
 
ClassId ClassName   StudentId   Subject     SubjectId 
1         ESL       12         English      20 
1         ESL       13         Science     30 
1         ESL       12         Social       40 
1         ESL       12         Maths        50 
 
Required output: parameters are Subject column values 

ClassId ClassName   TotalStudents   SubjectIds  
 
1        ESL                     1              20, 40, 50 
1        ESL                     1              30 
 
When one student takes multiple subjects then count student only once, so in the above data 12 is one student id takes multiple subjects so counted only once. TotalStudents value is 1
 
I did write below query: 
 
Declare @subjectids string 
set @subjectids = '20,30,40,50' 

-- will split @subjectids  and store in temp table    

select classname, classid, Count(Distinct StudentId) 
from mytable 
where SubjectsIds in @subjectIds 
group by ClassId, ClassName, SubjectId, 
 
but it gives me below output:   

ClassId ClassName   TotalStudents   SubjectIds 
 
1        ESL        1              20 
1        ESL        1              30 
1        ESL        1              40 
1        ESL        1              50