Yes the eligibility criteria for NEET states you need to have either passed +2 or appearing for it. If you are in +1 and want to take it just for practice then that's a completely different thing. But if you want to apply for a seat in a medical college and if you get selected, they will check all the documents and they need +2 mark sheet. 

Usually the application forms are available in January but exam takes place in may after board exams. So at the time of exam no one's gonna ask you about you being in 12(but you should). Now when you qualifies and goes for councelling and admission first thing you are asked is your 12th mark sheet.