Regarding accepting offer letter but still not signed the bond and get placement from college.

Hello Respected Authorities, Here I Want to ask that I have signed offer letter in which it is mentioned about the bond but still not signed any type of bond or not given any Certificates or Bond Cheque or not done the legal procedures for joining the company due to College Exam Schedule comes and joining becomes Delay and currently want to mention that I'm in 8th SEM Engineering and 7th sem exams are Ongoing that's why joining is delayed and now I don't want to join the Company so Still confused what to do What are the Procedures to do. Also to ask what to say to placement cell because the NOC for joining company and not to attend the classes on the other hand I want to ask whether not to joined this company is it necessary to join another company to show that why I'm leaving at last there are lot of doubts heading on mind what to do or not to do but one thing clear that not to join the company because I want to still study for master but due to at time of placement environment I'm placed but than realized about future studies. So what to do at this time, just want a mentor to discuss this stuffs and clear doubts.