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this works for my install:

$args  = array(
        'orderby' => 'meta_value',
        'order' => 'DESC',
        'meta_key ' => 'last_name',
        'role' => 'subscriber'
    );

so you have to tried:

$args  = array(
        'orderby' => 'meta_value',
        'order' => 'DESC',
        'meta_key ' => 'last_name',
        'role' => 'guest-teacher'
    );

and change the order parameter to DESC or ASC ?

ps: And you have the lastname filled out for these users?