Forced to pay school bus fee

Location/place: Patna

Name of company/service: DAV Public School, Indrapuri, Kesharinagar, Patna, Bihar

I availed the school bus service for my two children. Due to extremely poor and irregular service by the school bus I discontinued the same and used my own car for transportation. After 9 months when the result declared and admission started for next session I was informed to clear the bus fees dues to get further admission. I was forced to pay Rs.13500 for the 9 months period even though I have not used the school bus service. What is the legal option I have against the school authority.