Hello ,

Yes it is true you need to qualify JEE for BTech in IIT or NIT . But there are a lot of other examination through which you can take admission in BTech. .

Here is list of such examinations :-

https://www.google.com/amp/s/engineering.careers360.com/articles/state-level-engineering-entrance-exams/amp

It is not so that that if you give JEE than your marks in 12 will decrease . You can focus on both at same time , all the questions in JEE belongs to 11th and 12th class topic . Only difference is the level of questions are different and in JEE the level is higher while in boards you will get easy questions . The reason why some students who prepare for JEE get low marks in boards is they leave practice of writing in boards you have to write a lot , so apart from getting deep knowledge practice writing .

Now what should you do if you want to prepare for both at same time . You should read NCERT text book line by line for boards as you will not get any question out of book except for a few numericals .

For JEE you have to study same syllabus a little bit different for Advance , so you just need to practice questions as much as possible from Physics , Chemistry and Maths and give mock test on regular interval .

Hope my answer will help you .

Thank you .