You can directly apply to 12th if you have completed your 11th. For more details about admission in NIOS, visit the link - https://school.careers360.com/articles/nios-admission

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You can apply directly to class 12th though NIOS if you have completed your class 11th you need not to study 11th class again, you just need to provide the relevent documents at the time of admission.
Also if you have studied class 12th and can provide a TC that says you have studied 12th and were unable to qualify the 12th class exam then you can register yourself under the program ON-DEMAN EXAMINATION. This is for those students who failed to qualify the exams. In this program you can give your class 12th board exams in the comiing months (January, February, March) and get you result within 45 days and save your year.
For more details you can visit the official NIOS website.

Hope this helps.

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