void bubblesort(Student *a, int n)
{
    int i,j;
    for (i=0;i<n-1;i++)
        for (j=0;j<n-i-1;j++) {
            if (a[j+1]<a[j]) {
                Student tmp = a[j];
                a[j] = a[j+1];
                a[j+1] = tmp;
            }
        }
}