Hey there

Many students give NDA exam in 11th but the thing is NDA has saparate paper for maths which has questions from both 11th and 12th and the difficulty of paper is also good thus I'll suggest you wait and since NDA conducts exam two times a year you will have chance to sit for the exam two times one during 12th and other after your exams. Also giving NDA will not benefit you as 11th standard students are not eligible so even though you clear the exam you'll not be selected.

Check about NDA

https://www.google.com/url?sa=t&source=web&rct=j&url=https://competition.careers360.com/articles/nda-eligibility-criteria/amp&ved=2ahUKEwi-vYzpkv3kAhXLiHAKHdjCCZkQFjAAegQIBRAD&usg=AOvVaw1I0C9hFQPh7j-5oJDaK70E&ampcf=1

Good luck