Is really plagiarism what they do?

Is really plagiarism to search in Internet for information? And if you copy the information from an encyclopedia? Nowadays, Internet is a very useful tool where you can search for information. What you can’t do is copy and past a whole work that is in Internet. But if you look for information and with all that information you write the task,that's not plagiarism. The teachers should ask about the task once that the students write it, so they could see if the have learned something.