An extract from the 'Terms and Conditions' section of IRCTC:
"Student Concession :
Only I-tickets can be exchanged with student concession by presenting the I-ticket and the valid student concession order to the nearest Railway Reservation Office. The Student Concession tickets are permitted only in Sleeper Class (SL) and 2nd Sitting (2S). Presently it is not permitted in e-tickets. In I-tickets, in case a student purchases a fully paid ticket in advance and the ticket is delivered to him through courier and subsequently he get a student concession from his educational institution. It...
more... is possible for him to exchange his fully paid ticket with a "Student Concession Ticket" By going to the reservation office along with his journey cum reservation ticket and a request for exchanging the same with the student concession. He will surrender his original fully paid ticket and a student concession ticket with the revised fare (with same seat/berth number status) shall be issued to him. The difference of fare (paid and payable) less the clerk age charge of Rs. 2 0/- shall be refunded and shall be credited to his account electronically on the next day by IRCTC on receipt of refund file from the Railways."