get_user_meta value is stored within strange characters

The mistake is with this line:

   // Add the school ID as a meta data
   add_user_meta( $user_id, 'school_id', [$school_id] );

No need for the array brackets when you store the user meta, change it to:

   // Add the school ID as a meta data
   add_user_meta( $user_id, 'school_id', $school_id );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)