Go to the college and ask for concession form at the office.You may have to give an written application and submit.In return the college autorites will give you a form which bears your residential address,date of birth,age,roll no,starting station and class.Show it to the PRS counter at the station.Montly 2nd class is free of cost and 15% for quaterly 2nd class ticket.You will get concesion from the station nearest to your residence to the station nearest to your school/college.You have to show your concesion form within 3 days from the date college will give u.