Hello Sayan,
When a student fails in CBSE board he or she can apply to give the reappear exam whenever they will be organized. May be after some month of the result.He or she can apply for it and try to work hard in a proper way to get pass in this chance. He or she atleast clear the concept of important topics for which there are the chances to come in exam.Then he or she will easily get pass in this try.