Coaching classes not refunding amount paid

I'd joined a coaching institute and attended around 8/9 lectures only as my batch has started in last week. At the time of joining(admission) I'd paid half of the fees(i.e.40k as they asked me to do so) and now I want to cancel the admission for my personal reason so they're saying that only 50% (i.e.20k) will be refunded to you. So can I file a case against them? if yes, what are the winning chances as that inst. is quite a large one and powerful too? if low chances then will they file a defamation case against me?? {you can answer me via mail mostly.}