Below 50% in XII will it affect to get a good B-School ??
There are some top institutes like FMS Delhi, XLRI Jamshedpur etc who do not give high weightages to the academic scores. You can get admission in these institutes by scoring high in CAT and XAT respectively. There are some IIMs and other top B Schools who can consider you if you score high in your management entrance examinations.
Hence it is recommended that you should prepare and give your best shot in Management entrance examinations to increase your chances of selection.
All the best!
In order to get admission in good B School,There are various criteria for shortlisting and selection after clearing the exam with good score such as past academic records, Extra curricular activity, work experience etc.
Your academic records matters but it also depend on your performance during the interview. I would suggest you to focus only on the Exam Preparation to be able to maximize your score so that you may minimize the impact of past academic records.
Good Luck!