Hi.
Since you have just entered the 12th standard. It is advisable for you to only focus on your CBSE BOARDS right now. According to this year's as well as previous year's BOARDS exam, the physics and chemistry papers were really tough.
You can utilize your summer vacations for studying for the JEE MAINS but you also need to focus on the percentage you have to score in your boards exam. Further, after your exams are over you will have enough time to prepare for MAINS.
You can also take classes simultaneously if you are in doubt about your JEE score and want to gain confidence and practice. You can join institutes like Akash, Vidyamandir classes, etc. But always keep your 12th Boards a priority as the percentage really matters alot and you can change it once you have moved past it.
This one year is going to be tough and will require hard work from your side but the future is always bright.

All the best for your exams! Study Well.
Hope this helped.
Thank you.

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