After you have added a new class, you will be redirected to a form where you enter the class's students.
If you should need more rows, just press the Add more students link. If you have fewer students than the number of rows, just leave the remaining rows empty.
After you have pressed Save, each students will have a unique login token for the game. You can view these at any time by choosing this class in the sidebar or in the main menu under My classes. There, you can also generate new login tokens for your students if that would be necessary.
Once your students have gotten started in Grow Planet, you will be able to follow their achievements in detail. Read more about this here.