Dear Mikey, one of the eligibility criteria is that you should complete your 10+2 in all subjects to be eligible for JEE Mains. Having said that as even students pursuing their 10+2 and appearing for exams are also allowed, you are expected to produce the 12th standard certificate and marks list at the time of counselling and admission without fail. So if there are supplementary exams immediately and you can pass Maths in that exam, you are still good to go. So in short you can write JEE Mains but cannot take admissions.