hi,

see if you do not want to appear in exam this year, then you have to again take admission in 12th course as a regular mode and have to do all the registration process again.  however it would be counted as gap of year between education. you did not mention what is the reason behind that you donot want to appear in 12th exam, however due to covid 19 pandemic exam is getting late already, students also get extra time for preparation as well. however if your preparation of exam is really not satisfactory than it would be best to appear in exam next year. however do consult with your teachers and school authority to fulfill the necessary proceeding to appear in next year.