Private School Asking Fee Of Previous Academic Year (14-15)

Sir, I am a defense service personal (IAF) and posted to Sirsa, Haryana. My son is studying in Private school (The Sirsa School) since July 2013. My wife was also appointed in the same school as a teacher in the month of Aug 2013. My wife served for a duration of 10 months from Aug 13 to May 14. She resigned from school after giving one month notice period. No appointment letter was issued to my wife for the job. From school management it was told that you will be given staff concession in tution fee of your ward. She left the school on 31 may 14. we have submitted all the fee with due date. We have all the receipt of fee. On 05 Feb 15 school informed us to pay concessional fee which was given to your ward as a staff concession though your wife has already left the school. Once again we paid the amount 3375/- to the school same day. Today we received the message from school that you have to pay the full amount of fee from Aug 13 to May 14 because your wife served only for 10 months and concession is eligible once you are serving for more than 12 months. But on the same time no instruction/ guide lines was brought in our notice from school administration about this clause. My question is this can any school can ask fee of previous academic year, whence we have deposited the fee according to the fee book. please suggest the necessary guide lines as we are under stress.