Relieving letter – Conformation bonus return with interest

Dear Sir Our bank recruited 10 management trainees from different B-Schools during Nov-Dec, 2012. During campus placement they informed us that we will be entitled for a conformation bonus of Rs.1.5lakhs on successful completion of 12months in the organization. But we did not receive any offer letter at time of campus selection except a mail conformation. We joined the bank in June2013 and on the joining date we received our offer letter. To our surprise the letter says we have to pay back the conformation bonus of 1.5lakhs along with an interest of 13% p.a if we leave the organization within 12 months from the date of receiving the bonus. This was not communicated to anyone of us at the time of recruitment. At that point of time we were left with no choice but to sign the offer letter as had passed out of our respective b-schools. As promised by the company they paid us the said conformation bonus in July, 2014. Most of us (6) resigned for the company because of various reasons. Now the company has withheld our last month salary, entitled leave balance and our relieving letters asking us pay 1.5lakhs with interest @13% p.a . Is there a way to proceed legally against our employer… all 6 of us are ready to approach court and even the reaming 4 who are still working with the bank are willing to join us. During campus recruitment the company has hided all these hidden clauses. Regards