Unfortunately it is not possible to cancel your 12th form once submitted you can confirm it with your school too.you can reappear again the next year only and only then if you have a valid reason. But in this case you will lose your 1 year as you won’t be able to get admission in college without passing 12th board class.

You’ve wasted the whole year and now you think that by not appearing this year, you will definitely study for the next year then remove that thinking from your mind.start your preparation now it dont take that much time even 1 month is enough to pass the exam.

All the best for exams!!