I've done something a bit like this for "career days" for elementary school kids. I coded up a perl script that was basically something like this:

    #!/usr/bin/perl
    $score = $ARGV[0]

    if ($score > 90) {
        print "A";
    elsif ($score > 80) {
        print "B";
    elsif ($score > 70) {
        print "C";
    elsif ($score > 60) {
        print "D";
    } else {
        print "E";
    }

And I'd ask, "What do you think happens if you put in '85'? What about 10000? What about -30?" If the group I was talking to seemed pretty sharp on the uptake, I'd ask, "If you wanted to hack it so that people who got 95 got an E, and people who got 50 got an A instead, what would you do?"

The kids really seemed to enjoy that. Apparently they even told the teachers I was teaching them to hack programs -- a bit concerning for the teachers, but mission accomplished as far as I'm concerned.