C++ array indexing starts at 0, so:

Code:
for( i =1;i<n;i++)
{
    scanf("%d \t %d \t %d \t",&number,&number2,&number3);
    rollno[i] = number;
    sub1[i] = number2;
    sub2[i] = number3;x`
    total[i] = sub1[i]+sub2[i];
    avg[i] = (total[i] / 2) * 100 ;
    grade[i] = gradecal(avg[i]);
     
}
Is going to skip the first entry in all your arrays.