I like to treat it as a learning experience. The definiton has been addressed by others here, but I am talking about the personal ethical/more learning. I tell students to picture that they have worked really hard on something, and then someone else passses it off as his own. They would be upset! I also give them a little history of copyright laws, from the perspective of Charles Dickens since I am an English teacher. If you can get them to see it as stealing and feel bad about it, the lesson will better sink in.
I think that at the high school level a lot of students are not even sure what constitutes plagiarism. As teachers we need to be certain that they know exactly what it is. As someone else has said if we are working closely with the student as he constructs his paper we should be able to be certain they are not plagiarizing.
One other part of the ethical discussion that I think has to take place with students and instructors regarding plagiarism is the motivation that students have to plagiarize and what we do as instructors to either encourage or discourage it.
There are lots of instructors who look at their role as cops or investigators, being sure to root out and catch people and using them as examples, I know I found myself doing that early on.
But what also has to be addressed are issues like:
-are we assigning so much work that the student has to plagiarize to feel that they are going to get a decent grade?
-are we emphasizing a good grade over everything else and in doing so encouraging plagiarism?
-are we looking for limited and narrow types of writing that a student may not feel capable of producing so they look for outside help?
There are a number of things that have to be discussed in order for students and teachers to be on the same page in terms of expectations and behaviors.
This is a challenging question. From the student's point of view, I think that students need direct and clear instruction as to what constitutes plagiarism and how it takes away from the classroom community of scholarship. If a classroom setting is meant to idolize and worship ideas and the construction of thought, then plagiarism flouts such realities because of its mercenary nature and the short cut to thought present. Students have to be instructed to intrinsically worship what is done in the classroom and assignments need to be created that reflect this uniqueness, making plagiarism that much more difficult. At the same time, I think that teachers could probably do more in terms of checking work in a process- based and incremental manner. If teachers are conferencing with students about their writing and seeing its development over time, there is less of a chance for plagiarism because students and teachers are working together on the construction of work products in a manner that emphasizes process over the deified and artificial product.