how to increase the values of varchar colum

Question

how to increase the value of varchar columns and assing the specific pattern

code is the cannot the increment

my store procedure is that

ALTER PROCEDURE [School].[GetRegisterationNumber]

AS

begin

declare @StudentRegNo nvarchar(50)

set @StudentRegNo=(select MAX(StudentRegNo)from School.StudentInformation)

if @StudentRegNo is null

begin

set @StudentRegNo='OXPS-0001'

select @StudentRegNo

end

else

begin

select 'OXPS'+'-'+RIGHT('0000'+ convert(varchar(10),@StudentRegNo),4)

--set @StudentRegNo=SCOPE_IDENTITY()

end

end