$VAR1 = {
          'score' => '26.5249035452273',
          'student_id' => 177,
          '_id' => bless( {
                          'value' => 'something_unique'
                        }, 'MongoDB::OID' ),
          'type' => 'homework'
        };

##</code><code>##

use strict;
use warnings;
use MongoDB;

my $client = MongoDB->connect();
my $db   = $client->get_database('students');
my $coll = $db->get_collection('grades');

my $result = $coll->find( { 'type' => 'homework' } )
  ->sort( [ ( 'student_id' => 1 ), ( 'score' => 1 ) ] );  #multipleSort

my $previous = "1";

while ( my $doc = $result->next ) {

# print $doc->{'student_id'} . " " . $doc->{'score'} . "\n"; # debug 
    if ( $doc->{'student_id'} != $previous ) {

        # the following doesn't work....
        # $doc->delete_one( {'_id'=>$doc->{'MongoDB::OID'}} );

        $previous = $doc->{'student_id'};
    }
}