Ethical dilemma? You decide.

[Al Skierkiewicz]

Example: The same TEAM realizes that the transmission designed and built in the fall perfectly fits their need for a transmission to drive the ROBOT arm. They build an exact copy of the transmission from the original design plans, and bolt it to the ROBOT. This would be prohibited, as the transmission – although fabricated during the competition season – was built from detailed designs developed prior to kick-off.

This would seem to say it all in your case, substituting "code" for "plans". As Cory has said, a little examination of the CNC code will show that some improvements can be made in the code. You can move things around, make more efficient use of programming space, etc. The skills are already there, this could be a great opportunity for your students to practice and learn.