How can one prepare for board exams as well as the IPM exam conducted by IIM Indore?

The preparation depends completely on you. Opting for coaching classes is not necessary to do well in your examination. I have friends who have cleared the exam without any coaching at all. 

You should rather choose to invest your time in the upcoming board exams. It will not only help establish a strong foundation but will help you ace your personal interview. You should also focus on every subject, especially mathematics.

The reason why you should concentrate more on mathematics is that a reasonable amount of the aptitude test contains questions on mathematical ability. It needs more practice because the pattern of the questions cannot be determined. You must become compatible with the subject so as to get to the PI round. 

Following your boards, you should start reading newspaper articles, to stay updated with current affairs. This will help you in the PI session. It will also enhance your English communication skills. You can fetch the sample papers of the same on the website of the institute.