Wrong Data Coming from database - php

Question

I try to make student result management system. I face problem when I try to give position every student depend on their total obtain mark in exam. In my output I get 1st(obtain mark 448),2nd(obtain mark 444),3rd(obtain mark 445),4th(obtain mark 447), 15th(obtain mark 450),8th(obtain mark 449) But I don't find any problem in my code. It should be 1st(obtain mark 450),2nd(obtain mark 449)....
if (!function_exists('get_student_position')) {
function get_student_position($school_id, $academic_year_id, $class_id, $student_id, $section_id = null) {
$condition = " academic_year_id = $academic_year_id ";
$condition .= " AND school_id = $school_id";
$condition .= " AND class_id = $class_id";
$condition .= " AND student_id = $student_id";
if($section_id){
$condition .= " AND section_id = $section_id";
}
$ci = & get_instance();
$sql = "SELECT id, avg_grade_point, FIND_IN_SET( (avg_grade_point+total_obtain_mark),
( SELECT GROUP_CONCAT( (avg_grade_point+total_obtain_mark) ORDER BY avg_grade_point DESC )
FROM final_results ) ) AS rank
FROM final_results
WHERE $condition";
$result = $ci->db->query($sql)->row();
$rank = '';
if(!empty($result)){
$rank = $result->rank;
}
if($rank == 1){
return $rank.'st';
}elseif($rank == 2){
return $rank.'nd';
}elseif($rank == 3){
return $rank.'rd';
}elseif($rank > 3 ){
return $rank.'th';
}else{
return '--';
}
}
}