Sainik school is not compulsory to appear for NDA exam.

the lifestyle at Sainik school closely resembles with the training given at military academy therefore it will shape you in in military lifestyle and it will provide you and a upbringing which will help you to complete your training at the military school smoothly apart from that there is not anything more relevant to Sainik school.

Appearing in India is possible respective of your school of class 12th but the training at National defense academy is quite of and it is better if you are prepared well in advance for those tough period therefore Sainik school may help you in that aspect.