Convert Row value into Column value in SQL Server that query contains group by clause

Question

I am having table contains below records

Create Table Student_Marks(Student_Id int,Student_Mark int)

Insert into Student_Marks Values(1,50)

Insert into Student_Marks Values(2,60)

Insert into Student_Marks Values(1,40)

Insert into Student_Marks Values(2,70)

Insert into Student_Marks Values(1,60)

Insert into Student_Marks Values(2,90)

Required Output

Student_id  Student_Mark
1           50,40,60
2           60,70,90

i need a query and that query should contains group by clause also for another purpose.so please suggest a query.