Ranking students based on their total score

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PHP

MySQL

Greetings, Please i need a help on how to rank students based on their total scores, i have table where their scores are stored, therefore, i want to be able to determine the rank/position of the individual student based on his/her total score.

My Table:

student_regno| academic_session| academic_term| class_name| class_group| total_score
OUT20267 2016/2017 First-Term Primary 6 A 480
OUT20336 2016/2017 First-Term Primary 6 B 520
OUT20139 2016/2017 First-Term Primary 6 A 593

I want something like this when i fetch it:

OUT20267 = 3rd; OUT20336 = 2nd; OUT20139 = 1st and so on...

What I have tried:

$class_name = $conn->real_escape_string($_POST['h_class_name']);
$class_group = $conn->real_escape_string($_POST['h_class_group']);
$student_regno = $conn->real_escape_string($_POST['h_student_regno']);
$academic_session = $conn->real_escape_string($_POST['h_academic_session']);
$academic_term = $conn->real_escape_string($_POST['h_academic_term']);


$data = "SELECT total_score FROM student_compile_result WHERE student_regno ='$student_regno' AND class_name = '$class_name' AND class_group='$class_group' AND academic_session = '$academic_session' AND academic_term = '$academic_term' ORDER BY abs(total_score)";
$datas = mysqli_query($conn,$data);
$row = $datas->fetch_assoc();

//while(list($student_regno, $class_name, $class_group, $academic_term, $academic_session, $total_score) = mysqli_fetch_row($datas))

$student_rank=$row['total_score'];

echo $student_rank;