You're giving them too much credit, unless it's handwritten, they're probably copying and pasting. Plaigiarism is a serious issue, and could get them expelled from some high schools. It is also a black mark on your school record, and their permanent records should be flagged warning future teacher to be wary of this. If he feels that's too strong, he could at least mention that that's what would happen, but he's giving them a second chance.