Programmatically set Allowed Values in Select List dependent on another Select List value

I am using entity registration and I have a registration form where i want the user to select what district they are from and then based on their select, prepopulate a list of schools available in the following select list.

Goal:
1) Select the District you belong to
2) Then based on the District you chose, choose a School available in that District

Code I am using but its not working. The numbers being used in the case are the value keys of the districts.

function ces_custom_form_alter(&$form,$form_state,$form_id) {
    // if the list field exists for this form, alter it
	if($form_id == 'registration-form'){
		$district =	$form_state['values']['field_member_districts'];
		$school = '';
		switch ($district) {
			case "4450000":
				$school = array(
					'none'=>'None',
					'school1'=>'Deerfield Elementary',
					'school2'=>'Hatfield Public School'
				);
				break;
			case "Abington":
				$school = array(
					'none'=>'None',
					'school1'=>'Rowe Elementary',
					'school2'=>'Shelburne Falls',
					'school3'=>'Buckland School',
					'school4'=>'Greenfield School'
				);
				break;
			return $school;
		}
        $form['field_member_schools'] = array(
		    '#type' => 'select',
			'#title ' => t('School'),
			'#options' => $school,
		);
    }
}

I've also tried it this way:

function ces_custom_get_list_options() {
    $field = field_info_field('field_member_districts');
    $allowed_values = list_allowed_values($field);
    switch ($allowed_values) {
        case "4450000":
            return array(
                'none'=>'None',
                'john'=>'John',
                'paul'=>'Paul',
                'george'=>'George',
                'ringo'=>'Ringo'
            );
            break;
        case "Abington":
            return array(
                'none'=>'None',
                'school1'=>'Rowe Elementary',
                'school2'=>'Shelburne Falls',
                'school3'=>'Buckland School',
                'school4'=>'Greenfield School'
            );
            break;
    }
}
// this is hook_form_alter
function ces_custom_form_alter(&$form,$form_state,$form_id) {
    // if the list field exists for this form, alter it
    if (!empty($form['field_member_schools'])) {
        // set the allowed options for the list field
        $options = ces_custom_get_list_options(); // returns array of key/value pairs
        $form['field_member_schools']['und']['#options'] = $options;
        // if the saved node contains saved list options, select/check them
        if (!empty($form['#node']->field_member_schools['und'])) {
            $saved = $form['#node']->field_member_schools['und'];
            foreach($saved as $delta=>$option) {
                $form['field_member_schools']['und']['#default_value'][$delta] = $option['value']; // select/check
            }
        }
    }
}

Still no luck. Any thoughts? I have seen some examples but most show creating the entire form and it's fields in the module, but in my case the form is already created and so are the two fields I am using.