GMail does not use Folders in the same sense that most desktop email programs do. Instead, GMail uses Labels. Each email can no labels, or as many labels as you would like. Labels will typically show up in the left hand column of your default GMail view. When you click on the label, any email that has been assigned that label will be displayed in the list. So you can have one email in several different Labels without having to copy it into various folders and having multiple copies of one email You can setup as many labels as you would like in the Settings under "Labels." Another helpful feature is the "Filters" option in GMail. With Filters you can set up GMail to do specific things with specific emails. Say for example, every time you get an email from a particular person, you would automatically like to label that message and take it out of the inbox. You can set that up in Filter. Here is an example video showing some of the power of these options:
How you could use these features is up to your own imagination. At our school, all students have email addresses that end in "ppa-student.org" A teacher may wish to setup a filter where anytime an email arrives with "@ppa-student.org" in the address, it automatically receives a "Student" label, so that way they can go to that label to see all of the student emails. Or maybe you teach your students that they need to email you an assignment, and in their email they need to type "Class 4-3 Assignment", and then you setup a label for that class, and create a filter where anytime "Class 4-3 Assignment" shows up in the email it automatically gets labeled for that assignment. It can make it easy to organizat your mail and be more efficient with yoru communication!