Students of one class can be divided into groups (to learn how to do that, please see here). When students from different groups do the same test, all their results end up on the same spot, in test results. To make it easier to check their grades (especially if different teachers are leading different groups) you can sort their test results by group:
Go to Teacher, Tests, and then to the test results.
In the upper right corner, you will see a drop-down menu with the name of your class. If you click on it, you will see your group names. Choose the group name to see their results.