Hello Akash,

Thanks for contacting Careers360.

No, it is not necessary to complete 11&12 maths syllabus in order to crack JEE Advance or ISI. There are many examples of aspirants who started preparing for JEE in 11 and succeeded. 

But starting preparation a bit earlier never harms. In fact it gives you an upper hand since it will give you extra time in 11&12 for practice and revision.

So, my advice to you is that if you can start early then it's very good. But if you are not able to do so due to some reason then don't panic. Focus on your Class 10 studies first. You will get ample time in class 11 & 12. You just have to utilise it well.

Hope this helps. Good luck!