Hi Krupesh,
First of all lets have a correct picture of the exam timeline - JEE Mains is held in 1st ot 2nd week of April, while JEE Advanced is conducted in the later week of May. Considering its the start of July, you have got 9 months to prepare for Mains and approximately 10+ months for the JEE Advanced.
Its good to know that your target is to be in the top 100. Basically, you need to clear JEE Mains to be eligible to appear in Advanced. So its imperative that you first focus on clearing the Mains. If you set your target high then the lower targets are achieved easily. Considering Advanced to be tougher than the JEE Mains, targeting JEE Advanced is a good strategy - keeping in mind you don't loose focus on the JEE Mains.
It doesn't matter which school board you are in, your score here matters - don't take school studies lightly. Whatever test materials and test series you have joined - you should be clear in the fundamentals of every topic, give them a thorough reading with a good understanding. Also, the tests that you appear for - take them seriously. Keep in mind the time limits, as your question solving speed with accuracy matters a lot. Don't let down if you score less, try harder the next time.
I hope this helps, reply if you have any further queries.
Best regards,
Harshad