Question

when we are writing a letter to our classmate shall we use yours truly at last

Answer 1

You can use a number of sign-offs. Some of them include 


Love
Yours Truly
Yours sincerely 
Sincerely 


But when writing a letter, the sign-off depends on your relationship with the receiver of your letter. If this classmate is close to you, then, yours truly would be the perfect fit.