Student writers' understanding of plagiarism

Today, I read Kate Kellen's comments on how her students don't understand the concept of plagiarism in writing. Just when do we teach students about this bane of the English teacher's existence? If elementary students are allowed to copy from the encyclopedia for reports and middle school students are allowed to copy from the Internet and other sources with no regard to summarization, paraphrasing, quoting and citing sources, is it then up to high school teachers to correct years of "damage"? I find that almost unbearable and sometimes impossible. The process of incorporating research into writing takes years of practice- not just four years of the sporadic paper in English classes. I wonder what college English teachers have to say on the topic.