Hello Anish !!! It is essential to perform NEET aspirants to perform well in board exam as well as obtain a good score in entrance exam for neet.Moreover,the concepts and topics are introduced in 9TH standard itself on the basis of stuff which we required for entrance exams conducted after 12th.So,I think its best to prepare for NEET in 11th onwards to minimize the pressure of balancing board exams  and neet preparation since the syllabus of the neet includes the NCERT curriculum class 11th and 12th combined.You just go through the link provided below it provides you the guidelines

https://medicine.careers360.com/articles/how-prepare-for-neet-along-class-12-board-exam

Hope the i might have helped you.

Good luck!!!