CBSE and NIOS are 2 different bodies and have 2 different guidelines. In CBSE, you attend School regularly. But with NIOS, you either have weekend classes because it is an open School. Those students who have completed all their CBSE exams will be assessed based on their performance in the exams. Those who appeared for more than 3 or more subjects the average of the marks secured in the best of the 3 will be given to those subjects for which exams have not been conducted. And those students who have written just 1 or 2 subjects, the results will be declared on the  basis of their internal, practical or project assessments for the subjects they haven't written the examinations.  And if students want to appear for the exams, they can do so when they are conducted. And NIOS has scheduled its exams in the month of August and it is too early to talk about NIOS exams cancellation. So we need to wait atleast untll July is over. But please keep checking the official website for more information.