The company that I work for has a 3-month notice period!?
Secure the admission to the B school mentioned. Resign from your position with the reason of higher studies. Be very specific. Using the admission letter, make a case for early relief from notice period. If they are reasonable, they will allow. If they are not reasonable, take a call - MBA or the job? If you are clear on leaving the job, leave it. Take the risk. Anyways, after MBA, your pre-MBA experience usually is not called into question. And if it is, explain the situation. You could keep a copy of your salary slips (from the beginning of your tenure till the end) and copy of all documents given by the company (joining letter, appraisal letters, promotion letters, etc.). In case, if they refuse to give you experience letter, you can use these documents to show your employment (if it comes to that) During all of this, do not lose your cool. Please remember, when it comes to policies and procedures, HR Executive has very little control (which is why they are inflexible). Be patient with them and take advice from them on what to do ahead. As a last resort, try to meet the higher-ups in the HR hierarchy and your own department. And if all fails, step 4.