Professionals should always cite sources. Passing off someone else's work as your own can end your career. Students also need to use proper quotation and citation, because the work belongs to someone else and that person deserves credit.
I always let my students know if you use someone else's words or ideas without giving them credit, you're plagiarising. This includes things you've previously written. Some students don't understand self plagiarism, but it's real.
Then when they are taking notes, I have them write down their information on note cards-word for word. This way when they look to the note card for material they know exactly what the information was, and they won't accidently plagiarize.
It seems that high school students think their teachers are idiots and can not recognize that they are turning in work that is not their own. Our district has each grade level do an extensive research paper. Every year there will be two or three students that will cut and paste something from a web site and claim it as their own. The above poster is correct, we have to be sure that students understand what it is then we have to have severe enough consequences for those that are caught.
I have also found that many students do not know what plagiarism is exactly. That is why I always take the time to explain to them what it is. It is certainly unethical to say that someone else's hard work is your own. Students who plagiarize need to be dealt with and suffer the consequences but at the same time it is important as a teacher to make sure that students know what it is exactly.
Dealing with plagiarism as a student may seem to be a cut and dry case, but quite often when students plagiarize material it is by accident. There are several ways in which a student can avoid plagiarism.
1- Read about a topic. Close the book and then write about the topic by memory instead of writing with the page open.
2- Ensure that any direct wording is put into quotes with the source attached.
3-Always document your source from where you attained your information.
4- Unless you are a genius, most information presented came from somewhere.
As far as a professional dealing with ethical issues one must always be aware that using information obtained from another source does not belong to one. Therefore, give credit where it is due by adding the reference and not copying the material directly.
Passing off another person's material as your own is definitely an unethical action.