I am sorry I am not familiar with term NIOS ?
Could care to expand it and explain it for me ?
Does it stand for National Institute of Open Schooling ?
The typical process for getting a verification letter for your 10+2 either from CBSE or ICSE or state syllabus or any other involves writing a written request to them for the verification and give them a copy of the DGCA flow chart and conversion application form .
Some authorities will give you the verifications like CBSE and ICSE ( gives it in one day i have heard )
Some state syllabus authorities wont send it to you and instead will send it directly to the DGCA instead .
In this case make sure you mention your file number and computer number on the written request and tell them to mention it on their letter to the DGCA also otherwise the DGCA will just use it wipe their desks clean
Make sure you get a received stamp on a copy of the written request and you can submit that as proof of verification
Make sure you do this as soon as possible as some authorities take their own sweet time in issuing the verification