Hello

Yes you can apply foe the nda exam while in 12th .But if you are going for nda you have to join a good reputated classes in july in the same year you are opt for 12th.,

You have to manage both 12th and nda preparation simultaneously.it required a good planning and capabilities to manage both at same time.

Don'trely totally on the nda exam because  it had a very low successful rate..

Proper preparation and management is the only key to mange both of them.

Feel free to ask

Good Luck....