Moodle has several ways for teachers to refresh their courses, including creating a fresh, new copy from the course backup file, or just resetting the course and purging all of the information. If you’re course has run and you want to reuse it with new students (not keeping any old student data) then the “reset” option in your course’s administration block is what you want to use.
Note that the reset option completely purges your course of forum posts, students, student submitted files and any other type of information that is not your course activities and resources (those will be left as is). Glossary items and entries can also be wiped out, so export any glossary items you want to keep before using the reset option.
For more information: http://docs.moodle.org/en/Reset_course