Add DB Table from wp plugin

plugins.php

<?php

register_activation_hook( __FILE__, 'plugin_activate' );

function plugin_activate() {
  global $wpdb;
  $table_name = $wpdb->prefix . 'wpems_class';
  $sql = "CREATE TABLE IF NOT EXISTS {$table_name} (
           `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
           `class_name` varchar(255) DEFAULT NULL,
           `class_numeric_name` int(11) DEFAULT NULL,
           `teacher_id` int(11) DEFAULT NULL,
          PRIMARY KEY (`id`)
        ) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1;";
  require_once ABSPATH . 'wp-admin/includes/upgrade.php';
 dbDelta( $sql );
}