我的Ajax POST有什么问题？

I'm trying to populate new select menu (#school) from db table based on previous selection (#region) with ajax. I wanna get something like that: If there's no result, then hide #school and #class and show only error message. If search. php found results, then show at first school, if school selected too, then show #class.

What's wrong with my ajax POST? it doesn't get anything from db. Please help me to get it work. my js looks like that

$(document).ready(function(){
    $("#school").hide();
    $("#class").hide();
searchSchool = function(regionSelect){
var selectedRegion = $("select[name*='"+regionSelect.name+"'] option:selected").val();
if (selectedRegion!='0'){
    $.ajax({
    type: "POST",
    url : "core/code/includes/search.php",
    data: {region_id: selectedRegion},
    success: function(result, status, xResponse){
        if (result!=''){
            $("#school").show();
            $("#class").show();
            $("#school").html(result);
            $("#error").hide();
        }else{
            $("#error").html("There is no school found in this region");
            $("#school").html('');
            $("#school").hide();
        }
    },
    error: function(e){
        alert(e);
    }
    });
}else{
    $("#error").html('Please select a region first');
    $("#school").html('');        
    $("#school").hide();
    $("#class").hide();
}
}
});

and search.php

<?php
require 'db.php';
if(isSet($_POST['region_id'])) {
$region_id = mysql_real_escape_string($_POST['region_id']);
$res=$db->query("SELECT * FROM schools  WHERE id='".$_POST['region_id']."'");
while ($row = $res->fetch_array(MYSQLI_BOTH)) {
echo '<option value="'.$row[0].'">'.$row[1].'</option>';
}
}
?>

It looks like:

$res=$db->query("SELECT * FROM schools  WHERE id='".$_POST['region_code']."'");

should be:

$region_id = mysql_real_escape_string($_POST['region_id']);
$res=$db->query("SELECT * FROM schools  WHERE id='$region_id'");

also don't forget to use mysql_real_escape_string(), as mentioned. Note that if you use the mysqli lib, you need to use mysqli_real_escape_string().

data: "&region_id="+selectedRegion, looks wrong to me. I would use data: {region_id: selectedRegion},.

Also, see @stewe's answer about using region_code instead of region_id.