get_user_meta value is stored within strange characters

by

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 );