gistfile1.txt

<?php

class ClassRoomAdmin extends ModelAdmin {
    static $managed_models = array('ClassRoom');
    static $menu_title = 'ClassRoomAdmin';
    static $url_segment = 'classroom-admin';
}

gistfile2.txt

<?php

class ClassRoom extends DataObject {

    static $db = array(
        'Date' => 'Varchar',
    );

    static $has_many = array(
        'Students' => 'Student',
    );
}

gistfile3.txt

<?php

class  Student extends DataObject {

    static $db = array(
        'Name' => 'Varchar',
    );

    static $has_one = array(
        'ClassRoom' => 'ClassRoom',
    );

    public function getCMSFields() {
        $fieldList = parent::getCMSFields();
        $fieldList->push(new DropdownField('ClassRoomID', 'ClassRoom', ClassRoom::get()->map("ID")));
        return $fieldList;
    }
}