You can also create a category only for your course with this setting. First of all you must be sure that "Force" checkbox is unchecked in general administrative panel for the option "Drop the lowest", this way you can set a different value in our own category for each course. So that you must go to gradebook and add a category for your course and than specify a custom setting for the field "Drop the lowest". It will work only for this course. For more details about the gradebook and grade category in moodle 1.9, you may checkout http://docs.moodle.org/en/Grade_categories. I hope it helps.
Gradebook
How to exclude lower grades from calculations
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