Go to the Teacher tab.
In the top right of this Teacher page, you will find two different dropdowns. The first allows you to switch between your classes. The second is the MANAGE menu. Click on that one.
You can use the "Copy Class" option to copy an existing class. First, you give your new class a name- make sure that it's different from the old class name to avoid confusion. Then decide which elements from the original class you want to copy:
Users: your new class will have the same students and teachers as the original class.
Users + Groups: your new class will have the same students, teachers and groups as the original class.
Course material: your new class will have the same course material as the original class.
Course material + Tests: your new class will have access to the same course material, as well as the tests or exams as the original class.