Woocommerce - set role of customer by coupon - wordpress

Question

In woocommerce: I would like to group customers when they make their first order.
I installed "user role editor" plugin to create specific role (Example: student, teacher, ...)
At his first order/register, the customer has to complete the coupon field to register.
The coupon content is for example "student-coupon"
How to set the role of the customer depending of the coupon?
I tried this method:
Group Woocommerce users by Coupon
... but i couldn't make it work...
Here is my code:
// ASSIGN GROUP FROM COUPON
add_action('woocommerce_thankyou', 'coupon_group', 10, 1);
function coupon_group( $order_id ){
$order = wc_get_order( $order_id );
foreach( $order->get_used_coupons() as $coupon_name ){
// Retrieving the coupon ID
$coupon_post_obj = get_page_by_title($coupon_name, OBJECT, 'shop_coupon');
$coupon_id = $coupon_post_obj->ID;
$coupons_obj = new WC_Coupon($coupon_id);
if( $coupons_obj->is_type( 'student' ) ){
//get the user id of the customer from the order
$user_id = $order->get_user_id();
/*
*
* logic that adds the user to the group cricket users
* i.e. $add_user_to_coupon_group = add_user_meta($user_id, 'custom_user_group', 'cricket_users');
*
*
*/
$add_user_to_coupon_group = add_user_meta($user_id, 'student', 'student_users');
}
}
}
The coupon title is "student".
The "User Role editor" role is Student (ID student).
... Not sure if i have to change both 'custom_user_group' AND 'cricket_users' or only 'cricket_users' on the "$add_user_to_coupon_group" line
maybe somebody see a better solution?
Thank's